Updated dev guide to include feedback from previous tech review

Changed UTF-8 to plain-text under call sequence
Updated Content Type header to payload_content_type
Rewrote call sequence descriptions for clarity
Changed title caps to sentence caps
Removed cq-devguide.wdl, which was added by mistake

Author: Constanze Kratel

Change-Id: Iffeb69f2fbe8820fcb3f48190316b1f510787a3d
Merge "Updated dev guide to include feedback from previous tech review"