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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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