Elaborate on Iteration and Efficiency on engineering principles page

Why is this change being made?

This merge request adds an explanation of what Iteration and Efficiency means in the context of the engineering practice at GitLab.

See https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/3695+

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
• For transparency, share this MR with the audience that will be impacted.
  • Team: For changes that affect your direct team, share in your group Slack channel
  • Department: If the update affects your department, share the MR in your department Slack channel
  • Company: If the update affects all (or the majority of) GitLab team members, post an update in #whats-happening-at-gitlab linking to this MR
    • For high-priority company-wide announcements work with the internal communications team to post the update in #company-fyi and align on a plan to circulate in additional channels like the "While You Were Iterating" Newsletter

---