Migrate other wikis

[crykn]

The wikis of the other libGDX repos should be migrated as well:

• gdx-ai
• ashley
• gdx-controllers (?)
• fbx-conv (probably not worth it)
• gdx-pay (?)

Steps involved (see #65)

The old wiki markdown files were ported to Jekyll with the help of this code. For smaller wikis, doing those steps manually or with a custom script might be easier.

• The main step involved was fixing the wiki-style links ([[streaming-music]] -> [streaming-music](wiki/audio/streaming-music.md)) and, in the course of this, organising the pages in subdirectories.
• Similarly, the paths of assets located in the old wiki repo were changed to refer to their new location in /assets/wiki/.
• In addition, every wiki page now needs a frontmatter specifying a page title:

  ---
  title: Modules overview
  ---

[Frosty-J]

While the libGDX wikis should be prioritised, I'm wondering if it would be acceptable for third-party library wikis to be added, e.g. https://github.com/crykn/libgdx-screenmanager/wiki -> https://libgdx.com/wiki/extensions/third-party/libgdx-screenmanager/. Obviously should be cooperated with the respective library owners.

[crykn]

Tbh I'm not inclined to add third-party libs to the wiki. I think in the long run that would make the wiki too complex to properly maintain. But, as always, I'm open to other opinions on this!

[ice1000]

https://libgdx.com/wiki/start/setup

Gradle now supports 19, it doesn't support 20. Wiki needs to be updated accordingly.

[crykn]

This isn't really the right place to post this (next time please create a new issue or better yet a PR), but I'll change the https://libgdx.com/wiki/start/setup page once libgdx/libgdx#7130 is shipped in a release.