docs: use parser_yaml extension to handle Netlink specs

docs: use parser_yaml extension to handle Netlink specs

Instead of manually calling ynl_gen_rst.py, use a Sphinx extension.
This way, no .rst files would be written to the Kernel source
directories.

We are using here a toctree with :glob: property. This way, there
is no need to touch the netlink/specs/index.rst file every time
a new Netlink spec is added/renamed/removed.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>