feat: add changelog system

[jd]

This implements a new changelog system to the docs so it's easy to
browse it and subscribe via RSS.

Fixes MRGFY-5981

[mergify]

Merge Protections

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🔴 🤖 Continuous Integration

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• all of:
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  • check-success = lint
  • check-success = test
  • any of:
    • -head-repo-full-name~=^Mergifyio/
    • check-success=Cloudflare Pages
  • any of:
    • check-success = test-broken-links
    • label = ignore-broken-links

🔴 👀 Review Requirements

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• any of:
  • #approved-reviews-by >= 2
  • author = dependabot[bot]
  • author = mergify-ci-bot

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• #review-requested = 0
• #changes-requested-reviews-by = 0
• #review-threads-unresolved = 0

🟢 Enforce conventional commit

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Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?:

🟢 📕 PR description

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• body ~= .{48,}

[mergify]

@jd this pull request is now in conflict 😩

[mergify]

@jd this pull request is now in conflict 😩