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.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>