Summary:
This annotates the plugin api sufficiently to generate some documentation
for it. See https://github.com/gristlabs/grist-help/pull/139
Contains some small code tweaks for things that caused typedoc some
trouble.
Test Plan: manual inspection of output
Reviewers: jarek
Reviewed By: jarek
Differential Revision: https://phab.getgrist.com/D3342
