Edits and shuffles FWaaS content

Adds new FWaaS section to networking intro section. Adds overview
diagram. Includes operational FWaaS content moved from Advanced API
section.

Change-Id: I5b0a9182c25d624598d37172918ad8cd01d5f880
Closes-Bug: 1304745
Merge "Edits and shuffles FWaaS content"