Add documentation for calendar trigger offset (#22554)

* Add documentation for calendar offset triggers

* Add documentation for offset trigger in template

source/_docs/automation/templating.markdown

@@ -37,6 +37,8 @@ The following describes trigger data associated with all platforms.
 | `trigger.calendar_event.all_day`     | Indicates the event spans the entire day.                                                                                       |
 | `trigger.calendar_event.description` | A detailed description of the calendar event, if available.                                                                     |
 | `trigger.calendar_event.location`    | Location information for the calendar event, if available.                                                                      |
+| `trigger.offset`                     | Timedelta object with offset to the event, if any |
+
 ### Device
 
 | Template variable | Data |

source/_docs/automation/trigger.markdown

@@ -858,6 +858,8 @@ Calendar trigger fires when a [Calendar](/integrations/calendar/) event starts o
 much more flexible automations that using the Calendar entity state which only supports a single
 event start at a time.
 
+An optional time offset can be given to have it fire a set time before or after the calendar event (e.g., 5 minutes before event start).
+
 ```yaml
 automation:
   trigger:
@@ -866,6 +868,8 @@ automation:
       event: start
       # The calendar entity_id
       entity_id: calendar.light_schedule
+      # Optional time offset
+      offset: "-00:05:00"
 ```
 
 See the [Calendar](/integrations/calendar/) integration for more details on event triggers and the

source/_integrations/calendar.markdown

@@ -32,6 +32,8 @@ automation:
       event: start
       # The calendar entity_id
       entity_id: calendar.personal
+      # Optional time offset to fire a set time before or after event start/end
+      offset: -00:15:00
 ```
 
 ### Example Automation