Add documentation around using default Google application credentials,
along with a changelog extra that describes the feature and the
potential impact on existing restic uses (read: none).
This commit refactors the documentation according to my proposal in #1273
and the discussion I had with fd0 on IRC.
The bits from the manual that I could not immediately put into the new
structure are contained in manual_rest.rst Anything else is still there,
nothing has been deleted.
I changed the heading markup to follow the convention used in Python’s
Style Guide for documentation, this convention is explained in a comment
at the top of every file.
I also added a paragraph on installing restic on Debian.
