Format tutorial: Make the code belong to the items

Partially address https://github.com/ocaml/ocaml.org/issues/321
once the omd bugs https://github.com/pw374/omd/issues/60
and https://github.com/pw374/omd/issues/61 are fixed.
companies.md: fix Incubaid logo link

We had the img already. It just wasn’t linked to.
companies.md: add Psellos logo