MAINTAINERS: add netlink_yml_parser.py to linux-doc

The documentation build depends on the parsing code
at ynl_gen_rst.py. Ensure that changes to it will be c/c
to linux-doc ML and maintainers by adding an entry for
it. This way, if a change there would affect the build,
or the minimal version required for Python, doc developers
may know in advance.

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