docs: mtd: convert to ReST

Rename the mtd documentation files to ReST, add an
index for them and adjust in order to produce a nice html
output via the Sphinx build system.

It should be noticed that Sphinx doesn't handle very well
URLs with dots in the middle. Thankfully, internally, the '.'
char is translated to %2E, so we can jus use %2E instead of
dots, and this will work fine on both text and processed files.

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.

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