Proposal: Allow `/stable/` url fragment to give the user a choice of version

The current url schema presents itself as such:

`https://docs.godotengine.org/en/[version]/classes/class_array.html`

Where `[version]` could be any version of the engine, including `stable`.

The problem is, we have year's worth of tutorials, blog posts all over the net, QA answers, answers on Discord, Reddit, Facebook, comments under youtube videos, that all point at `/stable` while meaning **3.5**.

When making a google search, _even for a node that doesn't exist anymore_, you may find stale results:


This is of course worse than a 404, and costs a newbie several minutes (hours?) of confusion.

This is the source of several times per day confusion in the community. This is also a problem that can only get worse over time, if not handled.

The proposal is therefore:

1. Make all `/stable` URLs present the user with a version choice to all stable versions, and redirect them to the version they choose.
2. If possible, allow for custom redirection pages to allow for human intervention where necessary (e.g, redirect to KinematicBody or CharacterBody and warn of the change).
3. When the corresponding page does not exist, do _not_ lead people to a 404, but tell them the page they want does not exist, and allow them to be directed to the root of the doc for the doc they are looking for.
4. When necessary to default to a version, default to 4.x or the latest stable, but keep a (dismissable) loud banner at the top saying the user was redirected.