Better document how to use Coq from Node.js

[ejgallego]

Now that we are a bit more clear about the backend / frontend, we could indeed promote a bit more the use of Coq from node.

There is some stuff to be discussed on how to make the CLI / API more user-friendly, but indeed it seems there is quite a large demand for such a thing, so hopefully we can tweak the node packages a bit more.

Something that may indeed be nice about node.js is that we are not bound to the JS version of Coq, node could for sure use a native one.