docs: netlink: link to family documentations from spec info

To increase the chances of people finding the rendered docs
add a link to specs.rst and index.rst.

Add a label in the generated index.rst and while at it adjust
the title a little bit.

Reviewed-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20231129041427.2763074-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>