Added overview page to explain the role of Observability within the Monitor stage

Why is this change being made?

In this Slack conversation, @abellucci suggested adding this issue from @kbychu to the Handbook.

I've tidied it up, applied the style guide, and expanded on some of the descriptions.

I haven't yet linked it from other pages, as I'm still a bit scared of the Handbook!

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

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