doc: Split and expand coding style documentation

This patch expands the coding style documentation, splitting it
into two documents: the core style rules and extended guidelines.
Note that it does not redefine or change the coding style (aside
from section 4.6.2) - generally, it is only documenting the
existing style in more detail.

The aim is for the coding style to be more readable and, in turn,
for it to be followed by more people. We can use this as a more
concrete reference when discussing the accepted style with external
contributors.

Change-Id: I87405ace9a879d7f81e6b0b91b93ca69535e50ff
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
Signed-off-by: Petre-Ionut Tudor <petre-ionut.tudor@arm.com>
diff --git a/docs/process/index.rst b/docs/process/index.rst
index 9c12de8..1cb5354 100644
--- a/docs/process/index.rst
+++ b/docs/process/index.rst
@@ -8,6 +8,7 @@
 
    security
    platform-compatibility-policy
+   coding-style
    coding-guidelines
    contributing
    faq