docs: filesystems: convert proc.txt to ReST

This document has a nice format! Unfortunately, not exactly
ReST. So, several adjustments were required:

- Add a SPDX header;
- Adjust document and section titles;
- Whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Add table captions;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/1d113d860188de416ca3b0b97371dc2195433d5b.1581955849.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>