Separate out audiences for hiring

Why is this change being made?

As a team member, reading the hiring page was not clear since the page started with Interviewing at GitLab and described the experience from a a candidate perspective. However, a lot of the content then explained how to use Greenhouse and move the candidate through the interview process.

I think for reader efficiency and clarity its important that the page be broken by audience:

1. Candidate who is being interviewed
2. Team member who is interviewing

This is a first iteration at breaking up the page. Additional content should probably be moved, but I wanted to get this started as a first iteration.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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