Set up Docusaurus and migrate first pages

I had to jump a few hurdles but I got there in the end, and I'm quite happy with the result!

In order to migrate smoothly, I've set up the new Docusaurus site as a monorepo package with pnpm (cf. pnpm-workspace.yaml). Daiquiri's source code and existing styleguide remain where they are, at the root of the repo (the root is itself a package in the monorepo -- cf. https://pnpm.io/pnpm-workspace_yaml). So the changes are mostly self-contained, with just a couple of minor changes to Daiquiri's code for convenience.

Here's a quick peek:

Comments incoming to explain everything...