siren: document triggers & conditions (Labs feature)

[fdmarcin]

Proposed change

Migrate the siren integration docs to the split-page format for Labs triggers and conditions.

This PR:

• adds split trigger pages for siren.turned_on and siren.turned_off
• adds split condition pages for siren.is_on and siren.is_off
• updates the siren integration page to include trigger and condition listings
• preserves the existing inline action documentation and adds integration-level automation examples

Type of change

• Spelling, grammar or other readability improvements (current branch).
• Adjusted missing or incorrect information in the current documentation (current branch).
• Added documentation for a new integration I'm adding to Home Assistant (next branch).
  • I've opened up a PR to add logos and icons in Brands repository.
• Added documentation for a new feature I'm adding to Home Assistant (next branch).
• Removed stale or deprecated documentation.

Additional information

• Link to parent pull request in the codebase:
• Link to parent pull request in the Brands repository:
• This PR fixes or closes issue: fixes siren triggers & conditions #44934

Checklist

• This PR uses the correct branch, based on one of the following:
  • I made a change to the existing documentation and used the current branch.
  • I made a change that is related to an upcoming version of Home Assistant and used the next branch.
• The documentation follows the Home Assistant documentation standards.

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