Adding guidance around shifting OKRs in the Product OKR Page

Why is this change being made?

Suggesting guidance around shifting OKRs per this slack thread: https://gitlab.slack.com/archives/C04T225LBEW/p1690820361250929

Outline a process to make OKR changes within a quarter, referencing our GitLab OKR guidelines on when that's appropriate, and:

1. Clarify WHO makes an OKR change: the assignee
2. Clarify HOW to make an OKR change: 
   1. Inform stakeholders
   2. Discuss and agree on tradeoffs
   3. Track the change in the parent objective’s description
   4. Update the SSOT
3. Clarify that OKR scoring should then be done on the latest version of an OKR.

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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