Remove older versions of the docs and have the latest (v12) be the only documentation.

[jderochervlk]

Older versions come up in search and are read by LLMs.

We've already setup v11.rescript-lang.org to maintain older versions of the docs, following a pattern used by tailwind and other libraries.

We will need to setup redirects.

[fhammerschmidt]

I would assign this to @cometkim if you don't mind?