tools: ynl_gen_rst.py: Split library from command line tool

tools: ynl_gen_rst.py: Split library from command line tool

As we'll be using the Netlink specs parser inside a Sphinx
extension, move the library part from the command line parser.

While here, change the code which generates an index file
to parse inputs from both .rst and .yaml extensions. With
that, the tool can easily be tested with:

	tools/net/ynl/pyynl/ynl_gen_rst.py -x -o Documentation/netlink/specs/foo.rst

Without needing to first generate a temp directory with the
rst files.

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