Adding package management to language overview

[RobertIsmo]

Hello!

added a Package Management subsection under build Zig Build System.

I think it would be a net positive for communicating language features if this was mentioned in the overview, even if early. However, taking into account concerns by @kristoff-it:

1. Not making package management its own section(even though I believe it eventually should be separate from the build system section in the overview). Rather, emulating the fact that it was a subsection of the build system in the release notes.
2. Minimal overview and instead linking to the release notes. With 0.11.0 being the debut that summarizes the feature well, Linking directly instead of making a separate overview.

[andrewrk]

I think I would rather write something different. Thanks for the contribution but I'm going to let this sit until I find time to write the section.

[RobertIsmo]

@andrewrk Hello, thank you for the quick reply. Do you still think it would be proper to remove or change:

It’s planned to have a Zig Package Manager, but it’s not done yet. One of the things that will be possible is to create a package for C libraries. This will make the Zig Build System attractive for Zig programmers and C programmers alike.

This being my original reason for open the pull request. since it contradicts:

The package manager is still in its early stages, but is mature enough to use in many cases.

[kristoff-it]

Closing in preparation to moving the website to Zine.

The general topic of how to document the package manager and where to put such documentation is still open. Since this PR was created a build system guide was added to the learn section which explains the build system up to (but not included) packages, but as I remember Andrew wants to move that page somewhere else anyway.