Add landing page to documentation

[ml-evs]

It would be nice to start using the-datalab.rtfd.io (I have asked RTD if we can drop the 'the') as a vanity URL (or if we can buy a better domain) that presents more motivational documentation to interested people. We can set this up through our current docs following e.g., squidfunk/mkdocs-material#1996.

Ideally we could design something that could share some style/branding with the "splash page" of a running datalab (cf #767).

A nice example is the documentation page for mkdocs-material itself: https://squidfunk.github.io/mkdocs-material/