Improve `loadPlugin` api docs to reflect usage with github repos

[jmetz]

The api docs here: https://imjoy.io/docs/#/api?id=apiloadplugin don't mention github or the syntax mentioned elsewhere ( https://imjoy.io/docs/#/development?id=generating-a-plugin-url-for-sharing) for how loadPlugin can be used to load a plugin from a github repo using just username/repo:plugin-name syntax.

There also seem to be no corresponding jsdoc (or requiring a larger refactor, typedoc or similar) method for generating the API section of the docs.