doc/start: update documenting-ceph branch names

doc/start: update documenting-ceph branch names

This PR updates the branch names in the
documenting-ceph.rst file. It gets rid of all references
to the "master" branch, and updates the language to
reflect the state of play in 2022.

inb4: This PR merely removes the most egregious inaccuracies,
the ones that were most readily evident on a cursory perusal.
The full text remains to be carefully read and fitted together
with care.

I had to start somewhere.

Signed-off-by: Zac Dover <zac.dover@gmail.com>