Docs: describe changelog entries in the pull request description

[SandObserver]

Summary

• Replace the changelog.d/ fragment instruction on the contributing page with the Changelog section of the pull request description.
• Drop changelog-fragments.js --check from the listed CI checks.
• Describe release prep as dating [Unreleased] instead of folding fragments.
• Note that the release app is a bypass actor on the main ruleset for changelog pushes.

Testing
All green. Text only, no visual change.

Checklist

• npm run build and npm run check both pass.
• Product UI is shown by running or capturing it, never recreated in CSS.
• Any new preview or screenshot carries no private hostnames or real data.
• Visual changes verified in both colour schemes and both docs themes.
• Links to the app repo still resolve.

[cloudflare-workers-and-pages]

Deploying stackyard-docs with    Cloudflare Pages

Latest commit:	7c1e92d
Status:	✅  Deploy successful!
Preview URL:	https://c96b6c75.stackyard-docs.pages.dev
Branch Preview URL:	https://docs-changelog-from-pr-descr.stackyard-docs.pages.dev

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