Add how we work details around UX ready and manager feedback
This MR has a few minor changes to clarify the Product Designer and Manager roles a bit. Key ideas to consider:
- The Product Designer should move an issue through the UX process and determines when an issue is UX Ready.
- Product Designers bring in UX Managers at appropriate times so the manager can provide meaningful feedback.
- UX Manager feedback is important for a few reasons, outlined in MR.
- It's encouraged for a Product Designer to collaborate with their team before marking an issue as 'UX Ready'.
- When something is forced into UX Ready that we feel misses the mark, we document that via 'UX Debt' (I couldn't find this explicitly documented anywhere).