Categorise specification index page (#335)

docs/specs/readme.md

@@ -2,20 +2,26 @@
 
 # Specs
 
+## Device Compliance
 - [UDMI Compliance](compliance.md)
-- [Tech Stack](tech_stack.md)
-- [Discovery](discovery.md)
-- [Onboarding](onboarding.md)
+
+## Device 
 - [Messages](../messages/)
-- [Message Walk](message_walk.md)
 - [Connection Models](connecting.md)
+- [Discovery](discovery.md)
 - [IoT Gateway](gateway.md)
 - [MQTT Client](mqtt_client.md)
 - [Point Mapping](point_mapping.md)
-- [Proxy](proxy.md)
 - [Sequences](sequences/) (device-to-cloud API)
 - [Subblocks](subblocks.md) (core-to-apps API)
-- [Site Model](site_model.md)
+- [Log/Status Entries](entries.md)
+
+## Device Onboarding, Site Model, Metadata
 - [Metadata](metadata.md)
+- [Site Model](site_model.md)
+- [Onboarding](onboarding.md)
+
+## Cloud Environment
+- [Message Walk](message_walk.md)
 - [Tech Stack](tech_stack.md)
-- [Log/Status Entries](entries.md)
+- [Proxy](proxy.md)