update: add cf-r2-sdk example

[Myxogastria0808]

Summary

This PR adds a new example page documenting cf-r2-sdk, a community-maintained Rust SDK for Cloudflare R2.

The page introduces the SDK, explains when it may be useful compared to raw HTTP or aws-sdk-s3, and provides example usage for common R2 operations (upload, download, list, delete).

The SDK is explicitly marked as community-maintained and not officially supported by Cloudflare.

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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.

[Myxogastria0808]

In this case, Should I write changelogs?
If I should write changelogs, where do I write my changes?

[github-actions]

Hey there, we've marked this pull request as stale because there's no recent activity on it. This label is helps us identify PRs that might need updates (or to be closed out by our team if no longer relevant).