Add information about gir-rustdoc & CI templates to the book

[pentamassiv]

I am currently trying to get #1379 merged. Bilal mentioned that information about gir-rustdoc & CI templates could be added to the book. It is a good suggestion, but I'd like to not extend the scope or the PR since it already is a lot to read and review. I believe the information could be added later as well so this issue serves as a reminder.