docs: add example of checking a registered action permission

[RamiNoodle733]

Summary

Adds a concrete example showing how to check an action registered with register_actions() using datasette.allowed() and a resource instance.

Changes

• Added a new Checking a registered action subsection to docs/plugin_hooks.rst
• Included a short async example that checks "view-document" against DocumentResource(collection, document_id)

Testing

• uv run blacken-docs -l 60 docs/plugin_hooks.rst
• uv run cog --check docs/plugin_hooks.rst
• uv run pytest tests/test_docs.py -q

Fixes #2469

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📚 Documentation preview 📚: https://datasette--2659.org.readthedocs.build/en/2659/