GCP Artifact Registry download URLs must have /simple path

[pasunboneleve]

Summary

The current uv docs for Google Artifact Registry integration state that download URLs are expressed as

url = "https://<REGION>-python.pkg.dev/<PROJECT>/<REPOSITORY>"

I just found, by following the documentation, that this is incomplete. Instead, they should be

url = "https://<REGION>-python.pkg.dev/<PROJECT>/<REPOSITORY>/simple/"

in order to work. The trailing / is optional, but I also included it to conform to the standard used in the sections referring to other cloud providers within this same page.

This PR makes the necessary change to the documentation.

Test Plan

1. Create a GCP Artifact Registry repository.
2. Add it to a Python project using

export ARTIFACT_REGISTRY_PULL_TOKEN=$(
    gcloud auth print-access-token --project=<PROJECT>
)

export UV_INDEX_BUILD_USERNAME=oauth2accesstoken
export UV_INDEX_BUILD_PASSWORD="$ARTIFACT_REGISTRY_PULL_TOKEN

and

[[tool.uv.index]]
name = "BUILD"
url = "https://<REGION>-python.pkg.dev/<PROJECT>/<REPOSITORY>/simple/"

3. Assuming packages were added to that repository using uv publish, pull one of these using uv add <package>.

[jtfmumm]

Thanks for catching this!