docs/sphinx: Indent cleanup

docs/sphinx: Indent cleanup

Sphinx, its linters, and RST modes in common editors, expect 3 spaces of
indentation.  Some bits already conform to this expectation.  Update the
rest to match.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Lars Kurth <lars.kurth@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>