Added guide for packaging .NET apps

[gridlocdev]

This PR adds a step by step guide to building Flatpaks for apps built using .NET technologies such as Avalonia UI and Uno Platform.

Building > Guides > Dotnet

[gridlocdev]

It looks like gitlab.gnome.org is encountering some issues at the moment which are making the link checks fail in CI. I'll wait a bit and re-trigger the link checks with a small white-space commit.

[bbhtt]

It looks like gitlab.gnome.org is encountering

Don't worry about linkcheck

[gridlocdev]

I've added a few new commits including all of the noted changes above, please let me know if there's anything else I missed!

[bbhtt]

Thanks!