docs: generate permalinks correctly for rst2html output

docs: generate permalinks correctly for rst2html output

The rst2html output generates the links for headings in a slightly
different way than we do for docs written in HTML, so we must match
another scenario when generating back links.

rst2html will also use <h1> tags for both the document title and
the first level of section titles, so we must expand the matching
to allow for this too.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>