docs: fix reference to core-api/namespaces.rst

Fix a couple of dangling links to core-api/namespaces.rst by turning them
into proper references.  Enable the autosection extension (available since
Sphinx 1.4) to make this work.

Co-developed-by: Federico Vaga <federico.vaga@vaga.pv.it>
Fixes: fcfacb9f

 ("doc: move namespaces.rst from kbuild/ to core-api/")
Signed-off-by: Jonathan Corbet <corbet@lwn.net>