This updates the Review Banner docs to discuss the Quick Access actions,

discussing how they work and what actions are available by default.
Some bits of the Review Banner docs were updated while here to improve

navigation and anchoring.
The action extension docs have also been updated to discuss how to

create custom actions for inclusion in the list, with an example of a

custom one.
The screenshots on the page have also been updated to reflect both the

current color palette and the Quick Access UI.

Built the docs. Checked for any obvious errors.
Tested the screenshots on a standard and retina display.