Update Product Onboarding to reflect templates

Why is this change being made?

Per the first step in Product#3357 (closed), we are updating the handbook with the new recommended templates and process for PM onboarding.

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.
• Once this MR is merged please post an update in:
  • #whats-happening-at-gitlab
  • [#product][product-slack]
• Update the sample issues (around line 157) once folks start using this new template OR create a good sample issue and/or video.

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