docs(rl-next): add notes for curl-based s3

Author: lovesegfault

doc/manual/rl-next/s3-curl-implementation.md

@@ -0,0 +1,26 @@
+---
+synopsis: "Improved S3 binary cache support via HTTP"
+prs: [13823, 14026, 14120, 14131, 14135, 14144, 14170, 14190, 14198, 14206, 14209, 14222, 14223, 13752]
+issues: [13084, 12671, 11748, 12403]
+---
+
+S3 binary cache operations now happen via HTTP, leveraging `libcurl`'s native
+AWS SigV4 authentication instead of the AWS C++ SDK, providing significant
+improvements:
+
+- **Reduced memory usage**: Eliminates memory buffering issues that caused
+  segfaults with large files
+- **Fixed upload reliability**: Resolves AWS SDK chunking errors
+  (`InvalidChunkSizeError`)
+- **Lighter dependencies**: Uses lightweight `aws-crt-cpp` instead of full
+  `aws-cpp-sdk`, reducing build complexity
+
+The new implementation requires curl >= 7.75.0 and `aws-crt-cpp` for credential
+management.
+
+All existing S3 URL formats and parameters remain supported, with the notable
+exception of multi-part uploads, which are no longer supported.
+
+Note that this change also means Nix now supports S3 binary cache stores even
+if build without `aws-crt-cpp`, but only for public buckets which do not
+require auth.