Redirect /docs/... to /docs/<framework-from-localStorage>/...

[kylegach]

It would enable sharing a framework-agnostic link like /docs/get-started/install and having that redirect to the framework of the viewer's choice. Importantly, this would mean we could stop hard-coding react into the various URLs we link to across our sites.

We could also consider offering a setting to always use the localStorage framework, even when one is in the URL.