create a landing page and docs page about using cocalc as an interviewing tool

[williamstein]

It has repeated come to our attention that CoCalc -- with it's combination of course management and realtime collaboration and TimeTravel history and chat -- is actually kind of useful for companies for conducting technical interviews. Let's make this more explicit, since it's not at all obvious.

NOTE: This is not a ticket about developing any new functionality in cocalc. This ticket is only about opening the door to that possibility.