Docs: Renamed Google Now to Google Voice Actions

This commit contains the following changes:
- Renamed instances of Google Now to Google Voice Actions
- Removed the Intents fired by Google Now section as it is obselete
- Added link to the Google Voice Actions page
- Changed the voice-icon.png's reference link to the System Actions reference
  section on the Google Voice Actions page
- Revised the HTML comment and local CSS by replacing Google Now box with
  Google Voice Actions box.

Bug: 19501778
Change-Id: I3e4364aaceaff4f5bceb090dd4f8da548dfa5c30