docs: rework alternative indexes documentation

[mkniewallner]

Summary

Closes #9867.

Update alternative indexes documentation to use [[tool.uv.index]] and the associated environment variables instead of UV_INDEX.

This also globally reworks the documentation by:

• adding AWS CodeArtifact keyring example
• adding packages publishing examples for all providers
• making it more consistent for all providers

It might be best to show how to publish packages only once for all providers, but the publish URL usually being different than the URL used to retrieve packages, even if this duplicates things, it might still be more straightforward for users to see exactly what is needed for each provider.

Test Plan

Manually tested retrieving packages from AWS CodeArtifact and GCP Artifact Registry using both token and keyring.

Could not test:

• Publishing packages
• Azure Artifacts (not using it at all)

[zanieb]

Hey! Thank you so much for taking this on.

I gave this a quick look and made some stylistic edits to the Azure section 6a9b5e4

Before I apply them elsewhere, what do you think of those? I'm mostly editing for consistency with the rest of the documentation.

Are you interested in applying the same changes to the other sections, or should I?

[mkniewallner]

Before I apply them elsewhere, what do you think of those? I'm mostly editing for consistency with the rest of the documentation.

Are you interested in applying the same changes to the other sections, or should I?

Sorry for the delay, changes look good, thanks! I've applied them to the other sections as well.

[zanieb]

Thank you!