docs: meson.build: Prepare for use of identical code for XSLT processing of htmls

docs: meson.build: Prepare for use of identical code for XSLT processing of htmls

Meson unfortunately doesn't give us any means to share the code using
xsltproc to output HTMLs processed by our template. This means we will
have to resort to copy&paste engineering.

To make things simpler, let's use the same block of code in
docs/meson.build but also any of the subdirs which generate htmls.

This will be achieved by making it configurable and wrapping it in a
comment that instructs anybody editing it to keep it identical.

We need to be able to configure the template file used and installation
directory. The rest of the processing is same as we do in
docs/meson.build.

This code will then be copied to subdirs to refactor the current
approach used there.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>