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: NMauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/1d113d860188de416ca3b0b97371dc2195433d5b.1581955849.git.mchehab+huawei@kernel.orgSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -81,5 +81,6 @@ Documentation for filesystem implementations.
    omfs
    orangefs
    overlayfs
+   proc
    virtiofs
    vfat