docs: EDID/HOWTO.txt: convert it and rename to howto.rst

Sphinx need to know when a paragraph ends. So, do some adjustments
at the file for it to be properly parsed.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

that's said, I believe that this file should be moved to the
GPU/DRM documentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>