docs: page.xsl: Improve generation of paragraph anchor links

docs: page.xsl: Improve generation of paragraph anchor links

Use the 'parent' axis to check whether the parent is a div with
class='section' rather than looking for 'toc-backref' anchor to see
whether to generate one of the headerlink alternatives. Both hare
docutils-specific thus apply to docs generated from RST documents.

This adds the links for pages generated from RST documents which don't
have a table of contents (and thus lack the 'toc-backref' anchors) and
thus fixes pages such as hacking.html and news.html to have reasonable
links which can be shared.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>