Documentation/module-signing.txt: convert to ReST markup

- Fix identatio for the document title;
- remove its index;
- create a table for hash algorithm to be used;
- use quote blocks where needed;
- use monotonic fonts for parameters;
- adjust whitespaces and blank lines;
- Fix case on section titles;
- add it to the user's book.

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