docs: ABI: create a 2-depth index for ABI

That helps to identify what ABI files are adding titles.
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/e267b36ae7f32bab2a86f1da6b40bb3e62c877d4.1604042072.git.mchehab+huawei@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/admin-guide/abi.rst

@@ -3,7 +3,7 @@ Linux ABI description
 =====================
 
 .. toctree::
-   :maxdepth: 1
+   :maxdepth: 2
 
    abi-stable
    abi-testing