Add documentation for Holiday integration (#29802)

* Add documentation for `elevation` in service `homeassistant.set_location`

* Add documentation for Holiday integration

* Update source/_integrations/holiday.markdown

Co-authored-by: G Johansson <goran.johansson@shiftit.se>

* Give proposals on holiday calendar automations and add reference to upstream library

* Tweaks

---------

Co-authored-by: G Johansson <goran.johansson@shiftit.se>
Co-authored-by: c0ffeeca7 <38767475+c0ffeeca7@users.noreply.github.com>

source/_integrations/holiday.markdown

@@ -0,0 +1,32 @@
+---
+title: Holiday
+description: Instructions on how to use holiday calendars in Home Assistant.
+ha_category:
+  - Calendar
+ha_iot_class: Local Polling
+ha_release: 2024.1
+ha_config_flow: true
+ha_domain: holiday
+ha_platforms:
+  - calendar
+ha_codeowners:
+  - '@jrieger'
+ha_integration_type: integration
+---
+
+The Holiday integration allows you to create a calendar of holidays in Home Assistant for powering automations.
+
+It uses the Python module [holidays](https://pypi.org/project/holidays/) to incorporate information about region-specific public holidays.
+
+A calendar entity has a state and attributes that represent the next upcoming event (only). A calendar trigger is a much more flexible way to power automations with fewer limitations than using the entity state.
+
+{% include integrations/config_flow.md %}
+
+## Holiday calendar automations
+
+Individual holiday *Events* are what powers automations such as:
+
+- Turn on the Christmas lights at the *start* of Christmas Day.
+- Prevent your covers from being opened on public holidays.
+
+For an overview, refer to [calendar automations](/integrations/calendar#automation). You can use [calendar trigger variables](/docs/automation/templating/#calendar) in a condition or action such as the event `summary`, `description`, `location` and in other places.