docs: Simplify passing of 'href_base' XSL variable

docs: Simplify passing of 'href_base' XSL variable

Historically we had two top level XSL files for top level and nested
documents which only differ in what they pass for 'href_base' to the
main 'page.xsl' file.

We can instead pass the variable as argument from the build system so
that we have just one XSL file and also allow for more nested document
trees in the future.

The '404' page is special even with the current XSL way so we add a
special case for it.

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