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>
3 files changed