[VSA] Explain what each stage represents (FE)

Problem to solve

Currently in VSA there are no explanations of what each stage actually represents. For default stages you can check the documentation, and for custom stages you can click to edit them, but it would be very helpful to have easy access to explanations in-app.

Proposal

In group-level VSA, let's add the description text for start event (start_event_html_description) and end event (end_event_html_description) in a popover.

Mockup

See #300235 (comment 497062209)

More information will be added to the popover in a follow-up iteration.