[EUI+] [Spike] New docs versioning

[tkajtoch]

Summary

We need to research the best way to add docs versioning to the new documentation site based on docusaurus. The new website will host documentation for multiple libraries versioned independently and general documentation that should always be up-to-date no matter what library version is selected.

Acceptance Criteria

• There should be a document written documenting how the new versioning should be implemented and how we should store the older versions alongside the latest one
• The document should include performance details for the chosen solution

[github-actions]

👋 Hi there - this issue hasn't had any activity in 6 months. If the EUI team has not explicitly expressed that this is something on our roadmap, it's unlikely that we'll pick this issue up. We would sincerely appreciate a PR/community contribution if this is something that matters to you! If not, and there is no further activity on this issue for another 6 months (i.e. it's stale for over a year), the issue will be auto-closed.

[tkajtoch]

We've solved it by reusing some of the logic we already created for the old documentation website.

To summarize, we have a versions.json file that keeps track of all published @elastic/eui versions. This file gets automatically updated by our release CI script. We then read it in the VersionSwitcher component and render the list in site header.

It's a straightforward solution that works because our docs website builds are static and stored in separate (and isolated) directories - public releases follow the v<SEMVER> format and PR previews the pr_<PR_ID> format.