Update process for Kanban hybrid

Why is this change being made?

This MR implements the changes proposed in RFC: Update project management process to use a hybrid of Kanban gitlab-org/growth/team-tasks#147 (closed)

• Replaces the Product Development Timeline with a Project Management Process
• Removes milestone planning, milestone capacity, and milestone commitments
• Adds weekly refinement and monthly roadmap planning
• Explicitly outlines workflow stages
• Adds due dates section
• Renames PM initiatives to Product initiatives
• Minor formatting / grammar updates
• Removes the use of ~workflow::verification and instead adds the use of ~workflow::staging and workflow::production
• Make it explicit that prioritization of a product roadmap is the responsibility of a product manager. Every epic / issue on a product roadmap should have a priority.

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
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.

For help with failing pipelines reach out in #mr-buddies in Slack