[R2] Add example page for Cloudflare.NET.R2 community SDK

### Proposed changes

## Summary

I'd like to propose adding an example page for [Cloudflare.NET.R2](https://github.com/Alos-no/Cloudflare.NET), a community-maintained .NET SDK for R2 object storage.

## Motivation

The R2 examples section currently includes SDK examples for Go, Java, JavaScript, PHP, Python, Ruby, and Rust. For .NET developers, the only option is [aws-sdk-net](/r2/examples/aws/aws-sdk-net/), which requires manual S3-compatible configuration and workarounds like `DisablePayloadSigning = true`.

A dedicated .NET SDK provides:
- Native dependency injection and `appsettings.json` configuration
- Strongly-typed APIs with full IntelliSense
- Built-in resilience (retries, circuit breaker) via Polly
- Operation metrics for cost monitoring
- Comprehensive [SDK documentation](https://alos.no/cfnet/articles/r2/index.html)
- and more

## Proposed changes

Add a new example page at `/r2/examples/cloudflare-net-r2/` covering:
- Installation and configuration
- List, upload, download operations
- Presigned URLs
- Delete operations (single and batch)

**Draft PR:** https://github.com/Alos-no/cloudflare-docs/blob/5ccb751eef4e13d5e1cac33dd3954d5918a8808e/src/content/docs/r2/examples/cloudflare-net-r2.mdx

## Notes

- The page includes a note clarifying this is a community-maintained SDK
- Examples are aligned with the [SDK documentation](https://alos.no/cfnet/articles/r2/index.html)
- Follows the same structure as existing SDK examples (aws-sdk-net, boto3, etc.)

Happy to adjust the content or placement based on feedback.

### Subject Matter

Cloudflare .NET R2 SDK

### Content Location

https://github.com/Alos-no/cloudflare-docs/blob/5ccb751eef4e13d5e1cac33dd3954d5918a8808e/src/content/docs/r2/examples/cloudflare-net-r2.mdx

### Additional information

_No response_