Migrating docs to MusicStore

[timunie]

What does the pull request do?

Scope of this PR:

• fix or update to an existing sample
• add a new sample

What is the current behavior?

Checklist

If this is a new Sample

• Added a ReadMe-file
• Updated the landing page
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with a link to your sample in the related documentation page

In any case

• Spell-checking done
• Checked if all hyperlinks work
• Checked if all images are visible

Fixed issues

[timunie]

This PR is still WIP, but not a draft for the checks to run.

[timunie]

From my side, this ready for review. A preview can be seen here: https://github.com/timunie/Avalonia.Samples/tree/docs/migrate_music_store_docs/src/Avalonia.Samples/CompleteApps/Avalonia.MusicStore

We also have a PDF version for printing. It has almost 10 MB so not sure if we should upload it or not.