Add "root" component, update component docs links

[sritchie]

This PR adds a missing component and updates the docs to point to github, vs the old gitgud repo.

[ChristopherChudzicki]

Ah, thanks for the documentation update!

Questions: How should the Root component be used? I actually intentionally left Root out of the original components PR because "root" seems to behave a little differently from the other node types in MathBox.

In particular, mathbox.root is undefined:

const mathbox = MathBox.mathBox({});
console.log(mathbox.root)

whereas mathbox.point, mathbox.surface, etc all generate those node types.

I never really looked into this, but my suspicion was that <root> nodes are intended to be created only via MathBox constructor, and not by methods on nodes.

Have you used root directly?

[sritchie]

Ah, I haven't and did this blindly; I was setting up components for all primitives along with their docs in https://github.com/mentat-collective/mathbox.cljs and noticed that that one was missing. We can keep it out!