Add guides for using an FTP and GitHub setup in a fansub group

[LightArrowsEXE]

I have no real plans of writing the FTP section since I simply don't know how to set one up, but I did write one for basic GitHub use. This is intended to be eventually referenced to when I write about SubKt, which works best when combined with a Git setup. Not sure if it's a good idea to also include that in this PR, but I'll hear from you if it's better to make a separate PR for that or not.

[FichteFoll]

As I said on Discord, I like the idea of introducing git to sub groups, although it might not be the easiest entry. However, some things will need to be changed before I can merge this, outside of the docs not building.

[LightArrowsEXE]

b9c4a8a Bloody hell, it is. >:(
Changing the casing locally does not update it when I push it to GitHub, so idk if you know any fancy tricks to lowercase it.

[FichteFoll]

Github somehow marked this PR as read despite me not having read it since my last review, so I almost forgot about it. I'll see to it within the week.

[FichteFoll]

Merged manually with a few changes on top.