docs: Notify users of API v3 deprecation

[dgmccart]

After the ML competition ends, the old API (v3) will be deprecated. We need to notify users prior to deprecation so that users of the v3 API can migrate to using v4.

This issue is for tracking an exploration/implementation of what options are available in the docs site theme for notifying users (e.g. banner), and where else we should notify users (e.g. Portal UI, changelog, etc.).