extensions: Add Extensions section

[TheEvilSkeleton]

@allanday I'll need your help for some of the terminology and concepts.

Closes https://github.com/flatpak/flatpak-docs/issues/100

[TheEvilSkeleton]

/home/runner/work/flatpak-docs/flatpak-docs/docs/dependencies.rst:134: WARNING: unknown document: 'available-runtimes?highlight=.Debug'
/home/runner/work/flatpak-docs/flatpak-docs/docs/dependencies.rst:171: WARNING: unknown document: 'available-runtimes?highlight=.Locale'

How do I make it highlight text?

[bbhtt]

@TheEvilSkeleton Hey, do you still want to work on this?

[TheEvilSkeleton]

No. I was thinking of working on it it later, but I wouldn't mind having someone take over the MR

[bbhtt]

https://github.com/flatpak/flatpak-docs/pull/419

[TheEvilSkeleton]

Thanks a lot! I'm closing this one