Update and restructure pom.xml and add missing javadoc

[HeinrichAD]

This PR ...

• ... updates the pom.xml dependencies, except jetty.
• ... also add the thing every developer loves and hates at the same time: all missing JavaDoc comments.
• ... is one part of #189.

Changes

• Code
  • add JavaDoc to each class, method and property inside the main code (not test code)
• pom.xml
  • restructure pom.xml and use properties to control each artifact/plugin version
  • use plugin management to lock down plugins versions to avoid using maven defaults
  • update all artifacts/plugins to latest version, except jetty (this would come up with to much changes for a singe PR) This also closes #180.
  • set javadoc plugin configuration to fail on warnings to force developer to write javadocs mvn javadoc:javadoc
  • add versions-maven-plugin to easily detect updateable artifacts/plugins mvn versions:display-property-updates (also add a version report)
  • add url tag to GitHub pages
• GitHub workflow
  • add "Tests" workflow This will be triggered on every push and PR and runs
    1. maven livecycle and if successfully
    2. junit test against
       • embedded jetty server (currently deactivated, because its not runnable)
       • jetty docker image
       • tomcat docker image
  • add GitHub Pages This will be triggered if the "Main" workflow (triggered by tag creation) if sucessfully completed. It is also possible to trigger this workflow manually with workflow_dispatch. This creates the maven sites and published them to https://plantuml.github.io/plantuml-server/index.html
• Readme
  • add some more badges
  • license
  • latest tag (-> PlantUML version)
  • tests workflow status
  • github pages workflow status Can only be rendered after next sucessful tag creation! Alternative: trigger workflow manually
  • docker images sizes (latest jetty and tomcat version)

[arnaudroques]

@HeinrichAD It seems that last version (1.2021.13) is not published. Any idea ? Thanks!

[HeinrichAD]

@arnaudroques It should work like before.

on:
  push:
    tags:
      - "v*"

If a new tag is created and pushed, beginning with v, the pipeline should be triggered.

At the moment I can not find any new tags (see).

[arnaudroques]

At the moment I can not find any new tags (see).

Sorry, my mistake ;-) Thanks for your answer !