Update EP team handbook page with test intelligence section

Why is this change being made?

Re gitlab-org/quality/engineering-productivity/team#133 (closed)

Goal

Per request by the E-group and the ~"group::pipeline insights" team, let's document our test intelligence/fail fast testing/test selection strategies in our team's handbook page. This is aimed to demonstrate to the wider community our existing approach to improve pipeline efficiency and our plan to release some of these configurations as GitLab features.

Other benefits

I found the communication around these pipeline configurations fairly misleading sometimes, due to names being used interchangeably in conversations based on the understanding of the audience involved. This documentation would be helpful to facilitate communication around these going forward with the consistent naming.

Do we want video?

This video was recorded by Albert 2 years ago. If that was helpful, should we record something again given that we might have more to share now?

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.
• 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
• 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.

---