Add react-static content for deploying docs

[kale-stew]

Description

This PR moves to deprecate spectacle-docs so that our docs are maintained and deployed from the main repo.

I'm more than confident I've overlooked something in this migration, so a second/third/fourth set of eyes will be very appreciated.

Closes #782

Type of Change

• New feature (non-breaking change which adds functionality)
• This change requires is a documentation update

Next Steps

• Add a CI step to deploy the docs build to an aws bucket to then serve up on the formidable.com site

[ryan-roemer]

There are a lot of references to the JS + MDX Spectacle boilerplate projects, but I think it's easier to wait until after this lands and then I'll hop in and update all the references to point to spectacle-cli with instructions. I've captured this here: #788

[boygirl]

I would also add a todo for adding a CI step to deploy the docs to surge staging on PR so you can check them out before merging to master

[ryan-roemer]

NIT: Can just make this internal function async and switch to fs.exists or if we're on a node that has deprecated it, we should add this simple util:

const { promisify } = require("util");
const fs = require("fs");

const stat = promisify(fs.stat);
const exists = (filePath) => stat(filePath)
  .then(() => true)
  .catch((err) => {
    if (err.code === "ENOENT") { return false; }
    throw err;
  });

[ryan-roemer]

Nice work! Feel free to ticket as appropriate for later for my existing comments.