feat(docs): add Google Artifact Registry index instructions (#8579)

## Summary

This commit adds Google Artifact Registry authentication instructions
for both basic HTTP authentication and keyring methods.

## Test Plan

Locally tested using both methods.

docs/guides/integration/alternative-indexes.md

@@ -62,6 +62,58 @@ $ # Configure the index URL with the username
 $ export UV_EXTRA_INDEX_URL=https://VssSessionToken@pkgs.dev.azure.com/{organisation}/{project}/_packaging/{feedName}/pypi/simple/
 ```
 
+## Google Artifact Registry
+
+uv can install packages from
+[Google Artifact Registry](https://cloud.google.com/artifact-registry/docs). Authenticate to a
+repository using password authentication or using [`keyring`](https://github.com/jaraco/keyring)
+package.
+
+!!! note
+
+    This guide assumes `gcloud` CLI has previously been installed and setup.
+
+### Password authentication
+
+Credentials can be provided via "Basic" HTTP authentication scheme. Include access token in the
+password field of the URL. Username must be `oauth2accesstoken`, otherwise authentication will fail.
+
+For example, with the token stored in the `$ARTIFACT_REGISTRY_TOKEN` environment variable, set the
+index URL with:
+
+```bash
+export ARTIFACT_REGISTRY_TOKEN=$(gcloud auth application-default print-access-token)
+export UV_EXTRA_INDEX_URL=https://oauth2accesstoken:$ARTIFACT_REGISTRY_TOKEN@{region}-python.pkg.dev/{projectId}/{repositoryName}/simple
+```
+
+### Using `keyring`
+
+You can also authenticate to Artifact Registry using [`keyring`](https://github.com/jaraco/keyring)
+package with
+[`keyrings.google-artifactregistry-auth` plugin](https://github.com/GoogleCloudPlatform/artifact-registry-python-tools).
+Because these two packages are required to authenticate to Artifact Registry, they must be
+pre-installed from a source other than Artifact Registry.
+
+The `artifacts-keyring` plugin wraps [gcloud CLI](https://cloud.google.com/sdk/gcloud) to generate
+short-lived access tokens, securely store them in system keyring and refresh them when they are
+expired.
+
+uv only supports using the `keyring` package in
+[subprocess mode](https://github.com/astral-sh/uv/blob/main/PIP_COMPATIBILITY.md#registry-authentication).
+The `keyring` executable must be in the `PATH`, i.e., installed globally or in the active
+environment. The `keyring` CLI requires a username in the URL and it must be `oauth2accesstoken`.
+
+```bash
+# Pre-install keyring and Artifact Registry plugin from the public PyPI
+uv tool install keyring --with keyrings.google-artifactregistry-auth
+
+# Enable keyring authentication
+export UV_KEYRING_PROVIDER=subprocess
+
+# Configure the index URL with the username
+export UV_EXTRA_INDEX_URL=https://oauth2accesstoken@{region}-python.pkg.dev/{projectId}/{repositoryName}/simple
+```
+
 ## AWS CodeArtifact
 
 uv can install packages from
@@ -118,4 +170,4 @@ uv publish
 
 ## Other indexes
 
-uv is also known to work with JFrog's Artifactory and the Google Cloud Artifact Registry.
+uv is also known to work with JFrog's Artifactory.