Add Plan OKR Drafting steps

Why is this change being made?

See https://gitlab.com/gitlab-org/plan/-/issues/854#note_1291833499

This MR is an attempt to put some guidance down for OKR hierarchies in Plan. It is intended to:

• Reduce the overhead in creating and measuring the progression of OKRs for EMs and SEM.
• Result in simple, consistent, shallow OKR hierarchies that preserve progress roll-ups where those should happen.
• Encourage minimal numbers of assignees per KR (ideally a single DRI).
• Work around some of the shortcomings of the OKR MVC while Plan and the SEG resolve those.

It should encourage OKR hierarchies that look like this:

flowchart TD
    A[Plan Objective] --> B[Project Management KR]
    A --> C[Product Planning KR]
    A --> D[Optimize KR]
    A --> E[Knowledge KR]
    A --> G[SEM KR]
    A --> H[Principal Engineer KR]
    F[Plan Objective] --> I[Optimize KR]
    F --> J[Knowledge KR]
    F --> K[Product Planning KR]
    F --> L[Project Management KR]

And avoid those that result in 'orphaned' Objectives that aren't visible in roll-up calculations:

flowchart TD
    A[Stage-owned Objective A] --> B(Stage-owned KR)
    D[Group-owned Objective for A] --> S(Group-owned KR)
    D --> T(Group-owned KR)
    D --> U(Group-owned KR)
    A --> C(Group-owned KR)

Or those that result in unnecessarily complex hierarchies:

flowchart TD
    A[Stage-owned Objective] --> B(Stage-owned KR)
    A --> D[Stage-owned Objective]
    D --> E[Group-owned Objective]
    E --> S(Group-owned KR)
    E --> T(Group-owned KR)
    E --> U(Group-owned KR)
    A --> C(Group-owned KR)

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