docs: add ABI documentation to the admin-guide book

As we don't want a generic Sphinx extension to execute commands,
change the one proposed to Markus to call the abi_book.pl
script.

Use a script to parse the Documentation/ABI directory and output
it at the admin-guide.

Acked-by: Jonathan Corbet <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Link: https://lore.kernel.org/r/565148[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/admin-guide/abi-obsolete.rst

···
+ABI obsolete symbols
+====================
+
+Documents interfaces that are still remaining in the kernel, but are
+marked to be removed at some later point in time.
+
+The description of the interface will document the reason why it is
+obsolete and when it can be expected to be removed.
+
+.. kernel-abi:: $srctree/Documentation/ABI/obsolete

Documentation/admin-guide/abi-removed.rst

···
+ABI removed symbols
+===================
+
+.. kernel-abi:: $srctree/Documentation/ABI/removed

Documentation/admin-guide/abi-stable.rst

···
+ABI stable symbols
+==================
+
+Documents the interfaces that the developer has defined to be stable.
+
+Userspace programs are free to use these interfaces with no
+restrictions, and backward compatibility for them will be guaranteed
+for at least 2 years.
+
+Most interfaces (like syscalls) are expected to never change and always
+be available.
+
+.. kernel-abi:: $srctree/Documentation/ABI/stable

Documentation/admin-guide/abi-testing.rst

···
+ABI testing symbols
+===================
+
+Documents interfaces that are felt to be stable,
+as the main development of this interface has been completed.
+
+The interface can be changed to add new features, but the
+current interface will not break by doing this, unless grave
+errors or security problems are found in them.
+
+Userspace programs can start to rely on these interfaces, but they must
+be aware of changes that can occur before these interfaces move to
+be marked stable.
+
+Programs that use these interfaces are strongly encouraged to add their
+name to the description of these interfaces, so that the kernel
+developers can easily notify them if any changes occur.
+
+.. kernel-abi:: $srctree/Documentation/ABI/testing

Documentation/admin-guide/abi.rst

···
+=====================
+Linux ABI description
+=====================
+
+.. toctree::
+   :maxdepth: 1
+
+   abi-stable
+   abi-testing
+   abi-obsolete
+   abi-removed

Documentation/admin-guide/index.rst

···
    devices
    sysctl/index
 
+   abi
+
 This section describes CPU vulnerabilities and their mitigations.
 
 .. toctree::

Documentation/conf.py

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