Format :ref: and :mod: references in docstrings

This will allow other modules to reference the documentation.

https://docs.softwareheritage.org/devel/configuration.html is a documentation page which doesn't exist yet, but I will create it to document the various config blocks we have in various files (eg. 'storage:', 'scheduler:', 'objstorage:', 'journal:', ...)

Example usage: swh-scheduler!347 (merged)