Configure readthedocs to preserve all versions

Currently the following versions are on [readthedocs](https://readthedocs.org/projects/ert/):

- `latest`
- [`ubuntu-20.04-readthedocs`](https://ert.readthedocs.io/en/ubuntu-20.04-readthedocs/) (last built on [14 October 2021](https://readthedocs.org/projects/ert/builds/14990299/))

It seems like it would be nice to configure it to show `latest` and `stable`, and maybe to also keep point releases (in case users are stuck on older versions for some reason). Maybe we can also get rid of `ubuntu-20.04-readthedocs` unless it's there for a reason.

It should also be possible to add a "Description", which I think would help reassure people that they are looking at the right thing.

Start at the linked Read the Docs project and review its current version settings and build history, including the ubuntu-20.04-readthedocs version. Define which latest, stable, and point-release versions should remain visible, decide whether the old Ubuntu version has a purpose, and add a project description; done means the selected versions and description appear correctly on Read the Docs.

Written by the indexing model from the issue text.