Clarify hiring process

\cc @stanhu @pcarranza @balameb @jschatz1

I'm clarifying a bit what the "multiple interviews" means, and moving hiring process steps that are function-specific over to the page of those job descriptions.

@pcarranza Can you add a separate MR to describe the technical questionnaire you send out before the screening call even happens? And can you consider switching the order of those things, OR adding the tech questions to the application form?

@jschatz1 can you similarly add a description of what you ask / methods used in testing UX/UI applicants?

Also fyi for @brittanyr