RFE: Document how to debug

It would be nice to have a `HowToDebug.md` or even a branch with debug information and flag set.

A branch could have the `-DCMAKE_BUILD_TYPE=DEBUG` instead of `RELEASE` for example.
Or maybe just a documentation would be enough since, you would have to explain how to run
- `flatpak run --devel --command=sh org.musescore.MuseScore`
- `gdb --args mscore -d` (`-d` to get debug messages from Musescore https://musescore.org/en/handbook/3/command-line-options )

(Maybe it is possible to have `-d` by default ?)

Also having, this could help debuging Font issues
```
# Font issue debug https://www.freedesktop.org/software/fontconfig/fontconfig-user.html#DEBUG
# Values: 16, 128, 1024 give reasonable output. Value: 1, 2, 8, give huge output
- --env=FC_DEBUG=16
```

I could help to write things, and with the upcoming v7, it should be useful.

Also it would be super useful to explain how to write a patch. Currently, I cannot even find the source code, I guess it is removed after the build, but I am sure there is a Flatpak option to keep the source. Having the option ready in a "Debug branch" and a link to relevant flatpak documentation would be useful. (`--keep-build-dirs` in https://docs.flatpak.org/en/latest/flatpak-builder-command-reference.html ?)