doc: build separately with Meson install tag

doc: build separately with Meson install tag

This allows building and installing only the documentation, without
recompiling the whole project, using:

meson build -Denable_docs=true
meson compile -C build doc
meson install -C build --no-rebuild --tags doc

In Debian/Ubuntu the documentation is built separately from the binaries,
in a separate architecture-independent build job, so that it has to be
done only once, instead of once per supported architecture.

Signed-off-by: Luca Boccassi <bluca@debian.org>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>