Link to documentation for older versions

[jludvig]

I have to stay on plate version 36 for a limited time while we upgrade react to 18

I had some problems with the recent docs update being incompatible with v36, and I thought I had to use web-archive to browse the old docs until a google search revealed they can be accessed through https://v36.platejs.org/docs.

It would be great with a link, preferably in the form of a dropdown with links to old version docs.