docs: add a developer reference for .repo/ paths

Currently the only reference for these is the source which can be a
pita when needing to refer to something quickly.

Change-Id: I52baeb9a4935814cf99fa9a9b3102e8e46cddb0d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/253972


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>