[GuillaumeRx]

Explanation

Update the staged @metamask/auto-changelog README for the monorepo:

• remove the temporary migration notice;
• replace standalone setup, testing, release, and preview-build instructions with the monorepo contribution link.

Package introduction, installation, CLI usage, and API usage are unchanged.

References

• Follows Align auto-changelog with monorepo tooling #100

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them