Documentation/initrd.txt: convert to ReST markup

- use a quote blocks where needed;
- fix the chapter/section/subsection markups;
- use ``foo`` for monotonic;
- use .. note:: for /sbin/init file permissions;
- adjust whitespaces and blank lines;
- add it to the user's book.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>