Selective migration docs + changelog

[aninibread]

Summary

Screenshots (optional)

Documentation checklist

• [ ] Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• [ ] The change adheres to the documentation style guide.
• [ ] If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• [ ] Files which have changed name or location have been allocated redirects.

[github-actions[bot]]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/changelog/	@cloudflare/pm-changelogs, @cloudflare/pcx-technical-writing
/src/content/docs/r2/	@oxyjun, @elithrar, @jonesphillip, @aninibread, @harshil1712, @cloudflare/workers-docs, @cloudflare/pcx-technical-writing

[github-actions[bot]]

Preview URL: https://692ffa14.preview.developers.cloudflare.com Preview Branch URL: https://anni-selective-migration-r2.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/changelog/2025-12-04-r2-super-slurper-select-object-migration/	https://anni-selective-migration-r2.preview.developers.cloudflare.com/changelog/2025-12-04-r2-super-slurper-select-object-migration/
https://developers.cloudflare.com/r2/data-migration/super-slurper/	https://anni-selective-migration-r2.preview.developers.cloudflare.com/r2/data-migration/super-slurper/