Add on_announcement trigger to media_player doc

[gnumpi]

Description:

Added on_announcement trigger to media_player doc.

Related issue (if applicable): fixes

Pull request in esphome with YAML changes (if applicable): esphome/esphome#<esphome PR number goes here>

https://github.com/esphome/esphome/pull/6691

Checklist:

• [x] I am merging into next because this is new documentation that has a matching pull-request in esphome as linked above.
  or

• [ ] I am merging into current because this is a fix, change and/or adjustment in the current documentation and is not for a new component or feature.

• [ ] Link added in /index.rst when creating new documents for new components or cookbook.