Keep docs of previous Ocean versions available online

To consider:
- keep only last X versions
- list only major releases (pointing to latest minor/patch for each major)
- list only major.minor releases

See: RtD docs on [versions](https://docs.readthedocs.com/platform/stable/versions.html) and [automation rules](https://docs.readthedocs.com/platform/stable/automation-rules.html).

Read the linked Read the Docs versions and automation-rules pages first. Choose and document a retention and listing policy for Ocean releases, then verify that the selected previous versions remain available online.

Written by the indexing model from the issue text.