Revive and update Action Triggers docs for 2.5

This commit revives the missing Action Triggers docs section, last seen
in the 2.0 version of the docs. Also, several small corrections and
wording edits were made, as well as a few actual feature changes since
version 2.0.  The most explicit of these feature changes is the addition
of a basic description for a new Event Definition field in 2.5: "Event
Repeatability Delay".

Signed-off-by: Remington Steed <rjs7@calvin.edu>