Update repo location for readthedocs

Debexpo has a readthedocs located here: https://debexpo.readthedocs.io/en/latest/building.html

It currently refers to the github repository. It needs to be switched to salsa.

TODO:

  • Switch url to salsa
  • Track live branch
  • Update docs to link the RTD url
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information