Update the definition/responsibilities/tasks for each role

Problem/Motivation

Follow up to #3367191: Speed up decision making: define the project lead as an arbitrator/tie breaker instead of ultimate decision maker →

There were several comments made in the MR of 3367191 and in comment #32 → that were bringing the details for the roles up to date. That was out of scope so they are moved to this issue.

Proposed resolution

TBA

Remaining tasks

Update the roles definition and responsibilities.
Consider adding the following paragraph, from #3367191-32: Speed up decision making: define the project lead as an arbitrator/tie breaker instead of ultimate decision maker → somewhere.

"The Product Manager decides which features, improvements or fixes should be prioritized based on various strategic factors. The Release Manager then makes sure these changes are smoothly rolled out to users. Together, they work to make sure updates to Drupal are stable and efficient, meeting user needs. Product management often determines what should be built and why, while release management determines how and when it gets released. "