Add guidance on communicating and documenting UX due dates

Why is this change being made?

To help UX and PM communicate efficiently about expectations and timelines. This was discussed in Growth Weekly meeting on March 10.

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what