MAINT: consolidate namespace

The documentation showcases the API endpoint of `vo.registry.search`, all the while, the top level namespace provides the same as `regsearch`.
I think this is a rather unfortunate and possibly confusing setup and we should instead aim to avoid providing multiple API endpoints for the same functionality.

OTOH, if people feel strongly for the need of `vo.regsearch`, then we should make that the official API endpoint and rework all the docs to use that, and only that.

Compare the documented vo.registry.search endpoint with the top-level regsearch and the proposed vo.regsearch endpoint. First resolve which single public API should remain, then update the documentation consistently and verify that only the chosen endpoint is presented.

Written by the indexing model from the issue text.