Migrate docs to Next JS / abandon Gatsby

[lavelle]

The docs for thumbprint.design are currently running on an old version of Gatsby.

I have attempted to upgrade to the latest version in #829, but ran into many issues:

1. Our Markdown plugins for extracting frontmatter no longer work on the latest version
2. It gives a cryptic error when attempting to load these files using allSitePage.
3. These errors prevent the docs from loading at all. This means we can't access the GraphiQL interface to see what the new API should be.
4. Gatsby also doesn't recognise our use of import styles for CSS Modules, instead requiring us to migrate every component to use import * as styles.

After spending a few hours on this, we decided that it's not worth the trouble, since as a company Thumbtack is moving everything to Nextjs anyway. As such, we should revive the work in #721, and migrate away from Gatsby.

cc @danoc for reference.