ci: add changelog check workflow

[laurentsenta]

Related Issues

Closes #12069

Proposed Changes

This PR adds a CI workflow that checks whether the PR contains CHANGELOG.md modifications.

The check can be skipped by adding [skip changelog] to the PR title or labeling the PR with skip/changelog. These instructions are included in the PR template and the workflow failure message.

The checks succeed if CHANGELOG.md is modified and fail otherwise.

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, build, chore, ci, docs, perf, refactor, revert, style, test
  • area, e.g. api, chain, state, mempool, multisig, networking, paych, proving, sealing, wallet, deps
• If the PR affects users (e.g., new feature, bug fix, system requirements change), update the CHANGELOG.md and add details to the UNRELEASED section.
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green

[rjan90]

Awesome!

[rvagg]

I can see this is going to be quite annoying, at least for a while