docs: sphinx: add a parser for yaml files for Netlink specs

docs: sphinx: add a parser for yaml files for Netlink specs

Add a simple sphinx.Parser to handle yaml files and add the
the code to handle Netlink specs. All other yaml files are
ignored.

The code was written in a way that parsing yaml for different
subsystems and even for different parts of Netlink are easy.

All it takes to have a different parser is to add an
import line similar to:

	from doc_generator import YnlDocGenerator

adding the corresponding parser somewhere at the extension:

	netlink_parser = YnlDocGenerator()

And then add a logic inside parse() to handle different
doc outputs, depending on the file location, similar to:

        if "/netlink/specs/" in fname:
            msg = self.netlink_parser.parse_yaml_file(fname)

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