Improve Engineering Principles handbook page

Why is this change being made?

This merge request attempts to improve our Engineering Principles handbook page:

1. Add a short introduction to the page, explain how our Engineering Principles are related to GitLab Values.
2. Add a bit more structure there, divide the page into sections: Iteration, Simplicity, Reliability, Velocity etc.
3. Reuse the content that had been on that page previously, to introduce changes in more focused (and easier to review) merge requests later.
4. Update an Iteration Principle section content in the least controversial way: explain why do we sometimes need a tailored efficient iteration plan, but do not make the "scooter heuristic" obsolete yet (as we presumably need a better replacement diagram first).
5. Make it a base for future changes, and iterate on the page in subsequent merge requests.

➡ Review app: https://docs-gb-new-engineering-principles-page.about.gitlab-review.app/handbook/engineering/development/principles/

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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