chore(plugins): remove documentation-plugin (repo 404)

[anistark]

The upstream repo danielrosehill/documentation-plugin returned a 404 during the weekly audit — it has been deleted or made private.

Summary

Type of change

• Add a plugin → please use the add-plugin template instead
• Update a plugin → please use the update-plugin template instead
• Remove a plugin → please use the remove-plugin template instead
• Marketplace / registry change (schema, validation, plugins/ tooling)
• Site / docs change (landing page, README, content)
• CI / workflow / repo tooling
• Bug fix
• Other

Checklist

• [x I have read and understood the contributing guidelines
• Changes are scoped and focused on a single concern
• Tested locally where applicable
• Updated relevant docs (README, site content, etc.) if behavior changed
• Did not hand-edit marketplace.json (it is generated)

[github-actions]

Plugin Validation ✅

Check	Status
Duplicate check	✅ success
Schema validation	✅ success
Marketplace build	✅ success

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