Documentation/applying-patches.txt: convert it to ReST markup

- use the correct markup to identify each section;

- Add some blank lines for Sphinx to properly interpret
  the markups;

- Remove a blank space on some paragraphs;

- Fix the verbatim and bold markups;

- Cleanup the remaining errors to make Sphinx happy.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>