docs: Add coding-style.rst

docs: Add coding-style.rst

This part represents the biggest chunk of the existing hacking.rst, and
despite that its utility is very limited because 'make syntax-check'
already guarantees most of the rules are followed over time.

Until the glorious day we finally codify our coding style completely
into a configuration for a tool such as clang-format and thus no longer
need a plain English description of it, move this part to a separate
page.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>