Verify contents and format of track documentation

[kytrinyx]

Each language track has documentation in the docs/ directory, which gets included on the site
on each track-specific set of pages under /languages.

We've added some general guidelines about how we'd like the track to be documented in exercism/exercism#3315
which can be found at https://github.com/exercism/exercism.io/blob/master/docs/writing-track-documentation.md

Please take a moment to look through the documentation about documentation, and make sure that
the track is following these guidelines. Pay particularly close attention to how to use images
in the markdown files.

Lastly, if you find that the guidelines are confusing or missing important details, then a pull request
would be greatly appreciated.

[BNAndras]

Using this as an opportunity to review all the docs in that folder.

ABOUT.md is very long and detailed. I'd suggest truncating the readme after we highlight the multiple compiler targets. The subsequent information can go into individual concepts.

INSTALLATION.md could highlight Haxe extensions (IntelliJ, Visual Studio Code, etc.) that might help students working offline.

LEARNING.md might need some pruning. We're highlighting the three sections in the Haxe Documentation Introduction, but those are readily accessible from following the Haxe documentation link. Perhaps we can change the first URL to "Office Haxe Documentation, Manual, and Docs" and drop the three URLs underneath it.

RESOURCES.md needs some more links. At the moment, we're only linking to the manual, but that's also in the LEARNING.md. We could add the Haxe Discord, the Haxe Discourse, and other such community resources here.

SNIPPET.txt is fine for the most part. I'd suggest formatting the code snippet though for consistency.

TESTS.md is fine.

The config.json is missing RESOURCES.md.