Docs: Upgrade to react-static v^7

[kale-stew]

We should upgrade to react-static v7 to reinstate hot-reloading to make doc development less painful.

There are a number of breaking changes we'll need to address upgrading from v5.9.12, including upgrading babel and deprecating our use of a couple plugins.

This should be considered a future improvement we can implement after the v6 release.

[boygirl]

@kale-stew urql recently migrated to react-static 7 and created a plugin to assist with creating docs routes from .md. You can pattern off that work: https://github.com/FormidableLabs/urql/tree/master/packages/site