docs: Updating GuidedStepFragment for 23.2, new action types

Adding docs for some updates for GuidedStepFragment and GuidedAction
done in support lib v23.2. Includes docs on date picker actions,
subactions (new subsection), button actions (new subsection).
Adding docs for popBackStackToGuidedStepFragment and
finishGuidedStepFragments convenience methods.
Also updating page keywords so this page has better search
discoverability.

Staged: https://android-dot-devsite.googleplex.com/training/tv/playback/guided-step.html

Bug: 26902886
Original SHA1: I792bbe6696cd144dbb6319b3c4eb44e67390217d

Change-Id: I57aeb9deb59bb2cdf5a062b24916b625e1553cf0