Moved Build phase 1 to outcomes and activities

Why is this change being made?

This change moves the Build Phase 1 - Plan from a detailed process description to an outcome and activity focused section. This means rather than listing how labels need to be applied it focuses on the why and the expected outcomes.

At a high level this phase focuses on two motions:

1. Issue breakdown and engineering review
2. Milestone prioritization process (process for PMs to prioritize issues, and EMs to accept)

Changes:

1. Focused the planning breakdown on MVCs and iteration
2. Eliminated all 12 major and 3 minor steps
3. Added a outcomes and activity table with links to the handbook
4. Emphasised the need for collaboration
5. Moved "ready-for-dev" and "deliverable" labels into this phase from Phase 2, along with description supporting the shift.

If teams feel the need for more detailed descriptions of how to perform certain tasks. e.g. the labelling of the testing workflow, I suggest adding this to the relevant handbook pages.

Note: We have removed the step by step in this MR but are considering linking to them as part of a 'sample flow on a different page.

Closes #9109 (closed)

Author Checklist

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