[VSA][BE] Explain what each stage represents

This issue takes care of the backend part of #300235 (closed)

Currently available texts:

• Event name
• Stage description (only for default stages

What's needed:

Better explanation of the events (start, end).

How:

Add an explanation method to each event class which provides the explanation text for the event. For a label based stage, let's include the label name and the color (nicely formatted, MD text) as well (if possible).

This explanation should be exposed as part of the initial stages API call, in the following serializer: Analytics::CycleAnalytics::EventEntity