Fix docs and add iQIYI to the company list

The documentation and the wiki are separate but both used.

* Documentation (the docs directory, published at https://mesosphere.github.io/marathon/docs/) is for end users of Marathon. It's for knowing how to use the API and run Marathon in production.
* The wiki (https://github.com/mesosphere/marathon/wiki) is for info about developing Marathon itself. It's intended and maintained largely for the team at Mesosphere that is working on Marathon, but it's open for anyone interested in modifying Marathon.