Support docs URLs defined in the driver registry index

[amoeba]

The current dbc docs implementation relies on a hardcoded mapping between driver paths and URLs but it should really be relying on what's in the driver registry index. Refactor the current doc docs implementation to look for a docs URL in the index.

The driver registry index doesn't currently have a specific field to hold the URL for a driver's docs so we'll need to implement the above feature in dbc but also,

1. Update the driver registry index YAML to include a field
2. Figure out where this field ultimately lives

For (2), since ADBC Driver Manifests don't have a richer structure for URLs, the best thing would be for us to standardize that. However, it might be a breaking change and I'm not sure it's worth it. Therefore, I think the docs URLs can live in the cdn-deploy codebase.