Create a main page for https://rust-lang.github.io/

[szabgab]

There are a number of projects under this URL e.g. https://rust-lang.github.io/rust-clippy/ and https://rust-lang.github.io/mdBook/ but https://rust-lang.github.io/ itself does not exist.

IMHO it would be nice to have a page there with links to some (all?) the sites on that address.