docs(commit-style): add commit style documentation

This change adds a new documentation page describing the commit style,
acceptable Conventional Commits types and scopes, and documents the
process for expanding the list of scopes.

Change-Id: Iad957b67fa71a879e8aa0790c58a5b08cec300d6
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/docs/process/index.rst b/docs/process/index.rst
index 37324b0..bba2b40 100644
--- a/docs/process/index.rst
+++ b/docs/process/index.rst
@@ -8,6 +8,7 @@
 
    security
    platform-compatibility-policy
+   commit-style
    coding-style
    coding-guidelines
    contributing