This ports the PDF manual by @catloversg into .md form, so it can be used in-game and via browsing on GitHub.
It also adds MathJax rendering to in-game docs, so that the math formulas can be rendered. (GitHub already supports this natively.)
It lets us bundle static files such as images.
Allows us to `imports img from "./img.png"` to retrieve an image's path.
Note that we'll have to add other entries in the global.d.ts file if we
want to handle other extensions than .png.
Adds mocks to Jest tests so that we don't fail tests when loading static
assets.
