Roll "intro" page into top nav item

[PGijsbers]

          I hadn't merged it yet since I was planning to check how to roll it into the "OpenML Server" item at the top of the list. Similar to "setup" on [this page](https://squidfunk.github.io/mkdocs-material/setup/).

Originally posted by @PGijsbers in #206 (comment)