docs: admin-guide: add a features list

Add a feature list matrix at the admin-guide.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/f6c1e366fbc7ce1c9c94c7dc6c7852c6377cc0be.1606748711.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/features.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+.. kernel-feat:: $srctree/Documentation/features

Documentation/admin-guide/index.rst

@@ -19,6 +19,7 @@ etc.
    sysctl/index
 
    abi
+   features
 
 This section describes CPU vulnerabilities and their mitigations.
 

Documentation/conf.py

@@ -39,7 +39,7 @@ needs_sphinx = '1.3'
 extensions = ['kerneldoc', 'rstFlatTable', 'kernel_include',
               'kfigure', 'sphinx.ext.ifconfig', 'automarkup',
               'maintainers_include', 'sphinx.ext.autosectionlabel',
-              'kernel_abi']
+              'kernel_abi', 'kernel_feat']
 
 #
 # cdomain is badly broken in Sphinx 3+.  Leaving it out generates *most*