Merge how-to section on S3 buckets into S3 store docs (#7972)

Rather than having a misc tutorial page in the grab-bag "package management" section, this information should just be part of the S3 store docs.

---------

Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems>

src/libstore/s3-binary-cache-store.md

@@ -2,7 +2,103 @@ R"(
 
 **Store URL format**: `s3://`*bucket-name*
 
-This store allows reading and writing a binary cache stored in an AWS
-S3 bucket.
+This store allows reading and writing a binary cache stored in an AWS S3 (or S3-compatible service) bucket.
+This store shares many idioms with the [HTTP Binary Cache Store](#http-binary-cache-store).
+
+For AWS S3, the binary cache URL for a bucket named `example-nix-cache` will be exactly <s3://example-nix-cache>.
+For S3 compatible binary caches, consult that cache's documentation.
+
+### Anonymous reads to your S3-compatible binary cache
+
+> If your binary cache is publicly accessible and does not require authentication,
+> it is simplest to use the [HTTP Binary Cache Store] rather than S3 Binary Cache Store with
+> <https://example-nix-cache.s3.amazonaws.com> instead of <s3://example-nix-cache>.
+
+Your bucket will need a
+[bucket policy](https://docs.aws.amazon.com/AmazonS3/v1/userguide/bucket-policies.html)
+like the following to be accessible:
+
+```json
+{
+    "Id": "DirectReads",
+    "Version": "2012-10-17",
+    "Statement": [
+        {
+            "Sid": "AllowDirectReads",
+            "Action": [
+                "s3:GetObject",
+                "s3:GetBucketLocation"
+            ],
+            "Effect": "Allow",
+            "Resource": [
+                "arn:aws:s3:::example-nix-cache",
+                "arn:aws:s3:::example-nix-cache/*"
+            ],
+            "Principal": "*"
+        }
+    ]
+}
+```
+
+### Authentication
+
+Nix will use the
+[default credential provider chain](https://docs.aws.amazon.com/sdk-for-cpp/v1/developer-guide/credentials.html)
+for authenticating requests to Amazon S3.
+
+Note that this means Nix will read environment variables and files with different idioms than with Nix's own settings, as implemented by the AWS SDK.
+Consult the documentation linked above for further details.
+
+### Authenticated reads to your S3 binary cache
+
+Your bucket will need a bucket policy allowing the desired users to perform the `s3:GetObject` and `s3:GetBucketLocation` action on all objects in the bucket.
+The [anonymous policy given above](#anonymous-reads-to-your-s3-compatible-binary-cache) can be updated to have a restricted `Principal` to support this.
+
+### Authenticated writes to your S3-compatible binary cache
+
+Your account will need an IAM policy to support uploading to the bucket:
+
+```json
+{
+  "Version": "2012-10-17",
+  "Statement": [
+    {
+      "Sid": "UploadToCache",
+      "Effect": "Allow",
+      "Action": [
+        "s3:AbortMultipartUpload",
+        "s3:GetBucketLocation",
+        "s3:GetObject",
+        "s3:ListBucket",
+        "s3:ListBucketMultipartUploads",
+        "s3:ListMultipartUploadParts",
+        "s3:PutObject"
+      ],
+      "Resource": [
+        "arn:aws:s3:::example-nix-cache",
+        "arn:aws:s3:::example-nix-cache/*"
+      ]
+    }
+  ]
+}
+```
+
+### Examples
+
+With bucket policies and authentication set up as described above, uploading works via [`nix copy`](@docroot@/command-ref/new-cli/nix3-copy.md) (experimental).
+
+- To upload with a specific credential profile for Amazon S3:
+
+  ```console
+  $ nix copy nixpkgs.hello \
+    --to 's3://example-nix-cache?profile=cache-upload&region=eu-west-2'
+  ```
+
+- To upload to an S3-compatible binary cache:
+
+  ```console
+  $ nix copy nixpkgs.hello --to \
+    's3://example-nix-cache?profile=cache-upload&scheme=https&endpoint=minio.example.com'
+  ```
 
 )"