Add missing docs: file tree

Problem to solve

We lack documentation about the file tree. (Kai noted this in #371903 (comment 1078519728).)

I assume we'd add it in https://docs.gitlab.com/ee/user/project/repository/ but I'm not sure what all I need to know?

Other links/references

Related to Features missing documentation in Code Review (#371903 - closed) where @phikai spotted the need a long, long time ago and I just found the to-do.