improve documentation about backends

improve documentation about backends

I noticed that, using just the man pages, there is no real description
of what backends are, or what ones are available. Except for some
examples.

Added a git-annex-backends man page, that is just a stub, but at least
describes what they basically are, and tells how to find the supported
ons, and links to the backends web page.

Sponsored-by: Brett Eisenberg on Patreon