Basic blocks layout for dashboarding

We've explored creating customizable dashboards in Explore dashboarding frontend capabilities with... (#366689 - closed), but it was using sample data.

Success criteria

• Set up the dashboard app/page - POC: Integrate basic dashboard layout with live... (!95657 - closed)
  • Create a new feature flag and put the new work behind it (we will turn on this project-level FF for the handbook project when ready)
• Create tie-in/follow-up mergeable MRs:
  • Adding the page to the projects side menu - Product Analytics dashboard basic structure (!98610 - merged)
  • Implementing blocks/gridstack - Product Analytics dashboard widgets structure (!98613 - merged)
  • Adding the two first blocks with charts powered by Cube