Review documentation/implementation of CI Rules vs Only/Except

As part of recent discussions on confusion with how rules works vs how only/except works, I am creating this issue to discuss how to improve our documentation of the features.

A common issue is doubled pipelines, caused by a Branch Pipeline and a Merge Request pipeline being spawned at the same time, this did not happen with only/except, which is a common occurrence with rules, especially when people are switching from only/except to rules. Often, just "translating" from one to the other is not enough, and this is causing a lot of confusion.

Related to &2783