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: fcfacb9f8374 ("doc: move namespaces.rst from kbuild/ to core-api/")
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/conf.py b/Documentation/conf.py
index 3c7bdf4..fa2bfcd 100644
--- a/Documentation/conf.py
+++ b/Documentation/conf.py
@@ -38,7 +38,7 @@
 # ones.
 extensions = ['kerneldoc', 'rstFlatTable', 'kernel_include', 'cdomain',
               'kfigure', 'sphinx.ext.ifconfig', 'automarkup',
-              'maintainers_include']
+              'maintainers_include', 'sphinx.ext.autosectionlabel' ]
 
 # The name of the math extension changed on Sphinx 1.4
 if (major == 1 and minor > 3) or (major > 1):