doc: Migrate CodingStyle wiki page to Sphinx

Move the current CodingStyle wiki page to doc/develop/codingstyle.rst.
The changes here are for formatting or slight rewording so that it reads
well when linking to other Sphinx documents.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>