Create Engineering Principles page

Why is this change being made?

The main Engineering handbook page is very long. This extracts one set of items with a common theme: our engineering principles. This covers topics such as:

1. Prioritization.
2. Code quality.
3. Iteration.
4. Collaboration.

It does not reword any content or change the structure beyond moving to a separate page yet.

New principles page: https://create-engineering-principles-page.about.gitlab-review.app/handbook/engineering/principles/

New index page: https://create-engineering-principles-page.about.gitlab-review.app/handbook/engineering/

Part of chief-of-staff-cto#2.

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
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this change to the correct DRI(s)
  • 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 in 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.
• 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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