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 <[e-mail]>
Fixes: fcfacb9f8374 ("doc: move namespaces.rst from kbuild/ to core-api/")
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/conf.py

···
 # 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):

Documentation/kernel-hacking/hacking.rst

···
 
 This is the variant of `EXPORT_SYMBOL()` that allows specifying a symbol
 namespace. Symbol Namespaces are documented in
-``Documentation/core-api/symbol-namespaces.rst``.
+:ref:`Documentation/core-api/symbol-namespaces.rst <Symbol Namespaces>`
 
 :c:func:`EXPORT_SYMBOL_NS_GPL()`
 --------------------------------
···
 
 This is the variant of `EXPORT_SYMBOL_GPL()` that allows specifying a symbol
 namespace. Symbol Namespaces are documented in
-``Documentation/core-api/symbol-namespaces.rst``.
+:ref:`Documentation/core-api/symbol-namespaces.rst <Symbol Namespaces>`
 
 Routines and Conventions
 ========================