docs: split the CODING_STYLE doc into distinct groups

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

CODING_STYLE.rst

@@ -7,6 +7,9 @@ QEMU Coding Style
 Please use the script checkpatch.pl in the scripts directory to check
 patches before submitting.
 
+Formatting and style
+********************
+
 Whitespace
 ==========
 
@@ -205,6 +208,9 @@ comment anyway.)
 Rationale: Consistency, and ease of visually picking out a multiline
 comment from the surrounding code.
 
+Language usage
+**************
+
 Preprocessor
 ============
 
@@ -526,6 +532,9 @@ are still some caveats to beware of
     }
 
 
+QEMU Specific Idioms
+********************
+
 Error handling and reporting
 ============================