Add Dependabot config for npm dependencies

I checked and there's no `.github/dependabot.yml` here either, so dependency updates (Docusaurus, React, etc.) are fully manual right now. I'd propose:

```yaml
version: 2
updates:
  - package-ecosystem: "npm"
    directory: "/"
    schedule:
      interval: "daily"
```

Split out from #8.
