Keep old versions of docs when new major versions are published

### Description

When we publish new docs with major version changes, we loose a lot of information that is relevant for apps that are still on older major versions.

We need to keep old versions for docs for a while so apps on previous major versions can still easily read the documentation without looking in the github commit history.

### Additional Information

- How many major versions do we need to keep?
- How should we make this available?
- Use github releases? With some quick googling I found a [relevant blogpost](https://glebbahmutov.com/blog/versioned-doc-pages/) that could be useful?

### Tasks

_No response_

### Acceptance Criterias

_No response_

Start by resolving how many major versions to retain and how older documentation should be exposed, including whether GitHub releases or a versioned-docs approach fits. Done means users of previous major versions can easily access the corresponding documentation without searching commit history.

Written by the indexing model from the issue text.