docs: Adapt to semantic tag usage of docutils-0.17 and later

docs: Adapt to semantic tag usage of docutils-0.17 and later

Docutils-0.17 switched to using <main> instead of a <div
class='document'> and <section> instead of <div class='section'>.

To ensure that our pages still work we need to slightly adapt our XSL
stylesheet to select the <main> tag properly and adapt the CSS to also
select the 'section' element instead of a class and to apply to a <main>
tag with the appropriate names.

Docutils-0.17 also changed to use 'h2' for section heading instead of
'h1'.

Note the styles applied to the 'main' element with certain id can't be
made more universal by just applying to the id itself, as in certain
cases (e.g. 'documentation') we also have sections with that name. This
was previously ensured by also matching the 'document' class which would
make it equal to the 'main' element.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>