Improve the developer documentation

[choldgraf]

I've added a first pass at developer documentation here:

https://thebe.readthedocs.io/en/latest/contribute.html

However I think we are missing some important pieces in there. I wonder if @minrk, @rowanc1, @chrisjsewell, or @joelachance would be willing to confirm or add the following:

• Installation and local build instructions
• How to make a release for thebe
• An explanation of the JupyterLab API that we are using
• Any best-practices resources for using npm/javascript dev workflows

I would do it myself but I don't really know anything about javascript! 😬😬😬

[minrk]

added some more in #248 and the scaffolding for js API docs here, so we can have nice docs coverage like we are used to.

[rowanc1]

One of the things that I am not completely sure about, is which package we are pointing to, and when it was last released! I think we should be pointing to thebe on unpkg rather than thebelab - one was updated 2 months ago, rather than a year.

You can also deprecate thebelab on NPM if you like, and you can undo that as well:
https://docs.npmjs.com/deprecating-and-undeprecating-packages-or-package-versions

If puts a notice to users that shows up in console and in the top of NPM:

npm deprecate thebelab "🛑  `thebelab` has been renamed to `thebe`, you can install the new version: `npm install thebe` 🚀"

[choldgraf]

I didn't realize that there was a release of thebe on unpkg...if that's the case, we should definitely be pointing to it instead of thebelab 😬