MAINT: consolidate namespace

[bsipocz]

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.

[bsipocz]

#211 (comment)