Svelte.dev suggestion: version switcher

[rChaoz]

Describe the problem

Quite a few libraries are doing this. I think it would be great if there were a version switcher on the svelte.dev website, to allow changing between documentation for Svelte 4 and Svelte 5.
It would be especially useful for this update, where even for migration purposes many might benefit from access to the old docs. This would also allow for clean, new documentation and guide pages, 100% rune-focused, without having to worry that people will read Svelte 4 code and search in the docs for what they're seeing and not find it.

Describe the proposed solution

There are many existing methods and tools to implement this, but I think the best implementation would be something simple that doesn't affect the existing site architecture.

POC: #11602

Importance

would make my life easier

[rChaoz]

Update: I couldn't sleep so I implemented a version switcher real quick. It allows viewing the docs from the svelte-4 branch by simply appending ?version=v4 to the URL - check it out in the PR, which also contains an explanation on how it works.

The main idea is that I want some feedback on this feature, because if this would be welcomed, I'd like to properly implement it.