Iteration 3: updated growth epic refinement process

Why is this change being made?

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Previous updates on the refinement process:

1. Update Growth stage handbook pages (!15102 - merged)
2. Iteration 2: Update growth epic refinement process (!17082 - merged)

This is the iteration based on the thoughts shared here - https://gitlab.com/groups/gitlab-org/-/epics/19995#note_2945473305 - below is a Duo generated summary of the change.

1. Clearer refinement completion criteria: Instead of vague "discussions resolved," the process now requires at least 3 checkmark reactions on refinement threads to indicate completion, making it easier to know when a task is ready to move forward.
2. Better workflow separation: Added a new "planning breakdown" stage that sits between refinement and actual development work, giving engineers a dedicated space to volunteer for tasks before diving into implementation.
3. Improved team participation guidance: Updated instructions to clarify that engineers should actively monitor refinement threads and add checkmark reactions when satisfied, rather than just passively reviewing.
4. Documentation formatting improvements: Converted bullet points and tables to use consistent markdown formatting for better readability.

The changes essentially make the project workflow more structured and transparent, with clearer signals for when tasks are ready to progress through different stages of development. This should help reduce confusion about task status and improve team coordination.

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Commits

• chore: updated growth epic refinement process

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