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: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

build-aux/syntax-check.mk

@@ -2048,7 +2048,7 @@ exclude_file_name_regexp--sc_prohibit_readlink = \
 exclude_file_name_regexp--sc_prohibit_setuid = ^src/util/virutil\.c|tools/virt-login-shell\.c$$
 
 exclude_file_name_regexp--sc_prohibit_snprintf = \
-  ^(build-aux/syntax-check\.mk|docs/hacking\.rst|tools/virt-login-shell\.c)$$
+  ^(build-aux/syntax-check\.mk|docs/coding-style\.rst|tools/virt-login-shell\.c)$$
 
 exclude_file_name_regexp--sc_prohibit_strtol = ^examples/.*$$