Put docs back in this repository

[Murderlon]

We tried an experiment where we put the docs inside the uppy repository so we can version them with releases and run some commands in this repository to get the latest version tag and download those docs.

This became significantly more tedious than it's worth

• You can't live preview your doc changes anymore
• If you do want that, you have to create a new branch in uppy and push your changes. Then you have to manually change the downloadDocs command to point to your branch. For every change in the other repository, you have to go back here and run that again.
• It gets worse when you are using MDX components inside the docs because the components still live here, making the point above even harder again.
• Linting gets out of sync. It may pass locally in uppy, but then in fails in CI as it tries to run the linting from this repository.

Let's put it back.

Ref: transloadit/uppy#5527