Surface provider triggers in the operators and hooks reference

[subhramit]

Closes #29918

This completes the remaining part of the issue by surfacing provider triggers in the generated operators and hook reference.

The Sphinx extension now reads triggers entries from provider metadata and renders them in the generated providers summary docs. It also includes integrations that expose triggers even when they do not define operators, hooks, or sensors.
Have added a unit test covering the trigger-only integration case.

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: GPT-5.4 following the guidelines
All code changes (and also this description) were manually touched up and reviewed by me.

[boring-cyborg]

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