Docs: Versioning considerations

A few observations on the use of docs versions:

- `latest` (a pre-release version) is set as the default docs version. This causes the notification banner to be shown - however, the link in the banner links to the default version (i.e., by clicking on it, you are still looking at the pre-release docs).
- There is only one `stable` version. Going forward, this might confuse users, who cannot access the docs for previous stable releases.

I propose adopting the same versioning scheme as for lakeFS-spec, with `latest` as an alias for the latest stable version and set as the default. Pre-releases are labeled as `development` in that scheme.