docs: lcd-panel-cgram.txt: convert docs to ReST and rename to *.rst

This small text file describes the usage of parallel port LCD
displays from userspace PoV. So, a good candidate for the
admin guide.

While this is not part of the admin-guide book, mark it as
:orphan:, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>