Documentation: Document each netlink family

This is a simple script that parses the Netlink YAML spec files
(Documentation/netlink/specs/), and generates RST files to be rendered
in the Network -> Netlink Specification documentation page.

Create a python script that is invoked during 'make htmldocs', reads the
YAML specs input file and generate the correspondent RST file.

Create a new Documentation/networking/netlink_spec index page, and
reference each Netlink RST file that was processed above in this main
index.rst file.

In case of any exception during the parsing, dump the error and skip
the file.

Do not regenerate the RST files if the input files (YAML) were not
changed in-between invocations.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>

----
Changelog:

V3:
* Do not regenerate the RST files if the input files were not
changed. In order to do it, a few things changed:
- Rely on Makefile more to find what changed, and trigger
individual file processing
- The script parses file by file now (instead of batches)
- Create a new option to generate the index file

V2:
* Moved the logic from a sphinx extension to a external script
* Adjust some formatting as suggested by Donald Hunter and Jakub
* Auto generating all the rsts instead of having stubs
* Handling error gracefully
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>