docs: add ABI documentation to the admin-guide book

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 <[email protected]>
Signed-off-by: Mauro Carvalho Chehab <[email protected]>
Link: https://lore.kernel.org/r/5651482b06500e69a1acdf92152f90a203e6521d.1604042072.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <[email protected]>