update how we plan section

Proposal to update the language documented in our handbook around how we plan based on the retrospective issue and survey results which are all shared: Product#100 (closed).

Thanks @kencjohnston for collaborating on this!

Discussion when talking about implementation and communication plan:

• Cover with Engineering Managers, Product Managers
• Set a transition/cutoff time
• Adjust product process, we need to ensure we consistently have "milestone ready" issues for teams to work on
• Adjust product process to communicate priority (there are some best practices from Jeremy using labels and an issue to make issue boards better for this use case)
• Should we define milestone ready and groomed or other terms for increased fidelity of issues
• How are we tracking the success of this change?

cc/ @clefelhocz1 @edjdev @kencjohnston @ebrinkman