General cleanup, mainly of links (thanks to sphinx.ext.extlinks)

Add the sphinx.ext.extlinks extension and use it to shorten common
external links, mainly those to Jami's Gerrit and Jami's GitLab.

Also update various other links and references, and change several
remaining occurrences of 'Ring' to 'Jami'.

Lastly, add and use my SVG network topology drawing, which I had
originally drawn for my Jami talk[1] for LibrePlanet 2021.

[1] https://kelar.org/~bandali/talks/jami-empowers-users.html

Change-Id: I58189030b6fafcbed06b81f7b38553eb39494d60