Add help text

[koonpeng]

fixes #102.

[mxgrey]

The intent behind #102 is for the help text to be visible from inside the diagram editor. It looks like this PR doesn't provide that yet.

I imagine we could have something like an ( i ) icon in the node configuration widget that shows the help text when hovered.

Would it be feasible to use something like react-markdown to support markdown in the help text? That would be coherent with rustdoc's use of markdown, and I think it would be helpful for making the documentation as readable as possible.

[koonpeng]

I was going to submit the frontend changes in another PR, or do you prefer if everything is in this PR?

[mxgrey]

Since the backend changes are relatively small and inconsequential without the frontend feature, I think it makes sense to do the whole thing in one PR. That way #102 can be closed with just this PR.

[mxgrey]

In d8431cd I've added a concept of example configs where users can register a list of specific example config values with a description attached to each one to describe what behavior to expect. I think this will help users understand how to fill in config information, especially since we aren't leveraging the schema to guide the user yet. We could potentially allow the user to click on an example from the list and have it auto-filled into the config box.