Don't build HTML documentation in Architecture: any builds

Don't build HTML documentation in Architecture: any builds

With Autotools, we would have had to leave gtk-doc-tools in the
Build-Depends for gtkdocize, but with Meson we can demote it to
Build-Depends-Indep.

Promote docbook-xml and docbook-xsl to Build-Depends instead: they are
needed in all builds for the example programs' man pages. Previously,
they were pulled in by gtk-doc-tools.