Docs: add initial documentation style guide

Change-Id: Ib4b710feda98b97d484817b1fb59d5893edc3704
diff --git a/doc/source/developer/index.rst b/doc/source/developer/index.rst
index 986bbe4..7b16e9c 100644
--- a/doc/source/developer/index.rst
+++ b/doc/source/developer/index.rst
@@ -14,3 +14,4 @@
    drivers
    triggers
    testing
+   docs