Add Q4 Shared R&D OKR and Product Org OKR process/timeline to Product Handbook

Why is this change being made?

Related to Product#2669 (closed)

What this MR does:

• updates the Product OKR process for Q4, taking into consideration feedback received on the Q3 Product OKR pilot
• introduces Shared R&D OKRs, with approval from @sfwgitlab and @edjdev

What this MR does not do:

• propose changes to the existing Engineering team OKR process
• propose changes to the higher level existing company OKR process

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
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  • 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.
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• 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.

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