Description

Add a changelog feature to docs

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

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Tests

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Summary by cubic

Add a Changelog page to the docs and link it in the Getting Started sidebar. It introduces the December 2025 update covering Opportunities, cloud agents, and Slack/GitHub triggers, and removes the old /changelog redirect.

• New Features
  • Added docs/changelog.mdx with content and a YouTube embed.
  • Added image: docs/images/changelog/opportunities.png.
  • Updated docs/docs.json to include “changelog” in the sidebar and removed the previous /changelog → /reference redirect.

^{Written for commit 4c7bc7a. Summary will update automatically on new commits.}