Docs landing page

[LauraLangdon]

Currently the docs landing page is this, but we'd like it to be the intro page. Turns out this is impossible! 🙃 In Docusaurs the landing page has to live in src, and docs files have to live in docs.

For the same reason, the landing page can't display the sidebar because Docusaurus only displays the sidebar on pages in docs.

I think we can solve this by integrating the docs site with the website, like React Native does.