Document how a decision is made to remove someone from a 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 →

Make sure that how people are removed from a role is included in drupal-core.md, "How are decisions made?" section. This refers to the items that begin 'If the change is adding ...'.

Proposed resolution

TBA

Remaining tasks

Describe the process and update the doc as needed.