docs: build: Use proper path to file in 'contribute' link

docs: build: Use proper path to file in 'contribute' link

NEWS.rst is based in the root of the repository and 'hvsupport.html'
doesn't have a backing file which can be edited since it's fully
generated. Our 'contribute -> edit this page' link on the bottom of the
page is wrong in those cases.

Fix it by adding the contribute section only when there's a source and
base the 'source' of a html file in the root of the repository.

Along with that we need to modify the scripts/meson-html-gen.py script
to accept optional 'pagesrc' and the XSL template to skip the
'contribute' section when we don't have a source.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>