docs: move changelog into the docs sidebar section

[aru-amedeo]

Summary

• Creates docs/changelog.md with proper Docusaurus frontmatter, syncing content from the root CHANGELOG.md (adds 1.1.0 and 1.2.0 entries that the old page was missing)
• Adds a Changelog entry at the end of documentsSidebar in sidebars.js, mirroring the pattern used in the terraform-arubacloud-examples MkDocs nav
• Updates the footer link in docusaurus.config.js from /changelog to /docs/changelog
• Removes the outdated standalone src/pages/changelog.md

Test plan

• Run npm run build and verify no broken links
• Check that the Changelog entry appears at the bottom of the API sidebar
• Verify the footer "Changelog" link routes to /docs/changelog
• Confirm all three versions (1.0.0, 1.1.0, 1.2.0) are visible on the page

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