gcc-plugins.txt: standardize document format

Each text file under Documentation follows a different
format. Some doesn't even have titles!

Change its representation to follow the adopted standard,
using ReST markups for it to be parseable by Sphinx:

- promote main title;
- use the right markup for footnotes;
- use bold markup for files name;
- identify literal blocks;
- add blank lines to avoid Sphinx to complain;
- remove numeration from titles.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>