docs: update AudioCache explanation, migration guide, replace package READMEs

[Gustl22]

Description

• update AudioCache explanation (closes docs: more descriptive codes and information in Getting Started tutorial for AudioCache #1256)
• replace package README.md symlinks with files, and switched main README file to audioplayers package (chore: Use license file directly instead of symlinks #1433, pub publish is not picking symbolic links dart-lang/pub#2699)
• update migration guide to use Changelog
• update contribution guide
• update parity table

Copying Symlink targets for release should not be necessary anymore.

Checklist

• The title of my PR starts with a [Conventional Commit] prefix (fix:, feat:, docs:, chore: etc).
• I have read the [Contributor Guide] and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation and added dartdoc comments with ///, where necessary.
• I have updated/added relevant examples in [example].

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

[luanpotter]

I'm hoping github will understand a symlink

[Gustl22]

Seems like GH is showing it fine also for root, or do you have a special case in mind?
https://github.com/bluefireteam/audioplayers/tree/gustl22/update-docs

[luanpotter]

we should update the Support section based on the flame repo

[Gustl22]

Updated, but don't know if you want the badge also, and if want to credit community somewhere.

[luanpotter]

do we need to escape single underscore? I don't think I've ever seem that on other markdowns

[Gustl22]

Saw it here, but don't think it's needed: https://raw.githubusercontent.com/flutter/packages/main/packages/video_player/video_player_avfoundation/README.md

[luanpotter]

LGTM

[Gustl22]

@luanpotter @spydon I also now updated some of the contributing stuff (regarding the lack docs for setting up Melos, which is quite important I think). I know that there could be updated much more, like in flame. But I also think every new passage needs to be maintained. So I'd rather keep it simple and leave out the redundant stuff.