tools: docs: parse-headers.py: move it from sphinx dir

tools: docs: parse-headers.py: move it from sphinx dir

As suggested by Jon, we should start having a tools/docs
directory, instead of placing everything under scripts.

In the specific case of parse-headers.py, the previous
location is where we're placing Sphinx extensions, which is
not the right place for execs.

Move it to tools/docs/parse-headers.py.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/0f5ac2d704cffe9834e589b39549d2393e1237ef.1755872208.git.mchehab+huawei@kernel.org