Draft: Rename to job rotation and update handbook page

Why is this change being made?

This MR:

1. Updates the name of the program from internship to job rotation. I'm prosing this update because an internship often indicates that you're learning so that you can then be hired for the job at the end of the internship. This is not always the case at GitLab, and there are multiple learning goals that a team member might achieve from a job rotation other than a new role.
2. Clarifies and updates the documentation around the program and clarifies expectations to make it easier for team members to nominate themselves for a job rotation.

The essence of this program isn't changing - instead, the name is being clarified and the documentation improved so it's easier for team members to sign up.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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