docs: parse-headers.py: convert parse-headers.pl

docs: parse-headers.py: convert parse-headers.pl

When the Kernel started to use Sphinx, we had to come up with
a solution to parse media headers. On that time, we didn't have
much experience with Sphinx extensions. So, we came up with our
own script-based solution that were basically implementing a
set of rules we used to have at the Makefile.

Convert it to Python, keeping it bug-compatible with the
original script.

While here, try to better document it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/ae5cfa8dff37e280cc9493fc95a51cd0cc0ba127.1755872208.git.mchehab+huawei@kernel.org