Add Java 17 install instructions.

[Confectrician]

Fixes #384

[Confectrician]

@wborn please have a look. I can't request your review via ui currently. 🤷‍♂️

[wborn]

Can we already create similar openhab-docs PRs for OH 4?

[Confectrician]

Can you give me some hint what you want to prepare especially? The java instructions for the install sections?

[wborn]

The java instructions for the install sections?

Yes references to Java 11 would be a good start and openHAB 3 references will also need to be updated:

$ grep -REi '(jdk|java)[ ]*11|openhab[ ]?3'

administration/logging.md:The format and filename to store log settings has changed. It used to be `org.ops4j.pax.logging.cfg` in openHAB 2 and is `log4j2.xml` in openHAB 3.
README.md:# openHAB 3.x Documentation Project
README.md:This repository contains the documentation for openHAB 3.x.
README.md:The process below is subject to changes until the openHAB 3.x website become the production one.
developers/index.md:The different guides of this chapter assume that you are somewhat familiar with Java 11 and that you have a rough understanding of Git's workflow (e.g. "checkout", "branches", "push").
developers/index.md:1. Java JDK 11, for example from Oracle [Oracle JDK 11](https://www.oracle.com/java/technologies/javase-jdk11-downloads.html). On Windows: %JAVA% must be set.
developers/ide/eclipse.md:1. Install the Java 11 JDK if you did not have it installed.
developers/ide/intellij.md:- git, Maven, IntelliJ and Java 11 are installed
developers/guidelines.md:1. openHAB generally targets the long time supported Java 11 release.
installation/armbian.md:Select OpenHAB2 (old version) or OpenHAB3 with **SPACE** and press **ENTER**
installation/security.md:This is an important new requirment in openHAB 3.0 and later versions.
installation/security.md:This is not required prior to openHAB 3.0. You must add the following two directives underneath the `add_header` (in the `server` block) and `proxy_set_header` (in the `location /` block) items respectively:
installation/security.md:    # openHAB 3 api authentication
installation/index.md:Make sure that you have an up to date **Java 11** JVM platform installed on your host system.
installation/index.md:Please download and install the **Java 11** version of the JVM.
installation/index.md:Note that openHAB 3 will run under Java 11, and you _can_ use it with openHAB 2.X as well. But be aware that although developers are working hard to make this work, there might be problems with the oldest parts of openHAB 2.x, such as some of the v1 bindings, due to non-backward compatible changes in Java 11.
installation/linux.md:There is a package description for stable releases of [openHAB 3](https://aur.archlinux.org/packages/openhab3/) in the AUR.
installation/linux.md:git clone https://aur.archlinux.org/openhab3.git
installation/linux.md:cd openhab3
installation/linux.md:sudo pacman -U openhab3-3.0.0.M4-1-any.pkg.tar.zst
installation/synology.md:DSM 7 does not longer support Java 11 directly.
installation/synology.md:You either have to run openHAB in a Docker container or have to take care of a properly installed Java 11 runtime on your own.
installation/synology.md:You may also get Java 11 via [community package](https://synocommunity.com/package/java-11-openjdk) and go own with the manual for [older DSM Versions](#older-synology-diskstations-till-dsm-6).
settings/configuration.md:openHAB 3 has introduced a new unified user interface called MainUI. Almost everything that can be configured in openHAB can be configured through MainUI.
settings/addons.md:openHAB by default support the Basic UI as well as the HABPanel (which used to be the standard before openHAB 3).
configuration/index.md:openHAB 3 provides a _single_ graphical user interface to modify settings, to manage your components and rules, and to provide a UI for users.
configuration/index.md:openHAB 1 was text only, openHAB 2 added the general administrative web interface "Paper UI", and openHAB 3 replaced that with "Main UI".
configuration/jsr223.md:Note that prior to openHAB 3, script ordering was performed alphanumerically based on file path. This is no longer supported as of openHAB 3.
configuration/migration/index.md:title: Migration to openHAB 3
configuration/migration/index.md:description: Description of Beaking Changes and needed steps for a proper Migration to openHAB 3
configuration/migration/index.md:# Migrating from openHAB 2 to openHAB 3
configuration/migration/index.md:There are different approaches to get your openHAB 2 Environment upgraded to openHAB 3.
configuration/migration/index.md:Below you can find some general informations for the technical upgrade process to openHAB 3.
configuration/migration/index.md:If you are working with an [openHABian](https://www.openhab.org/docs/installation/openhabian.html) setup, the upgrade is quite easy. Regardless of if you are currently using the openHAB 2.5 stable release or one of the latest 3.0.0 SNAPSHOT or milestone builds, switching to openHAB 3.0.0 stable is done in just a few steps:
configuration/migration/index.md:1. When being asked to update openHABian, answer yes. Do not change the branch, it should read `stable` if you are on 2.X and `openHAB3` or `main` if on 3.X.
configuration/migration/index.md:Unfortunately, it means that in terms of openHAB 2 and openHAB 3, you cannot install both at the same time and openHAB 2 will not automatically update to openHAB 3, but if you use the commands to install openHAB 3 whilst openHAB 2 is installed, then the configs will copy across automatically.
configuration/migration/index.md:openHAB 3 requires Java 11. Most modern Linux OS have an `openjdk-11-jdk` package available for you to install but alternative solutions such as [Zulu's OpenJDK](https://www.azul.com/downloads/zulu-community/?version=java-11-lts&package=jdk) (which openHABian can install for you) are available and these may run faster on 32-bit armhf platforms such as the Raspberry Pi.
configuration/migration/index.md:You must also be on the relevant repository for openHAB, this hasn't changed for openHAB 3.x so the [same instructions for moving to it](https://www.openhab.org/docs/installation/linux.html#package-repository-installation) are still valid.
configuration/migration/index.md:If the install of openHAB 3 detects an existing openHAB 2 install, then it will copy the configurations  across and update using those files. There are several major differences between openHAB 2 and 3 which may mean that this will cause issues, let us know if you experience anything strange.
configuration/migration/index.md:Note that openHAB 3 now requires a Java 11 runtime, so please make sure to [install such a JVM](https://next.openhab.org/docs/installation/#prerequisites) before continuing.
ui/index.md:[**Layout pages**](./layout-pages.html), introduced in openHAB 3, are the most common and versatile way of displaying information in the main UI.
ui/building-pages.md:**UI components** are the basic building blocks for many UIs in openHAB 3.
tutorials/getting_started/pages_intro.md:openHAB 3 introduces a new unified user interface called MainUI.
tutorials/getting_started/pages_intro.md:openHAB 3 provides a number of options to supports this in addition to MainUI Pages.
tutorials/getting_started/model.md:openHAB 3 makes extensive use of the semantic model to automatically create Pages (see next section) and to provide natural language interaction.

[Confectrician]

I have added an issue for this.

Since we are on snapshot level for the latest website version and the milestones will also depend on java 17 we definetely should prepare this already. :)