Onboarding for new Fleet users

[mike-j-thomas]

Problem

Landing in Fleet UI can be overwhelming for new users. There is no guidance on what to do first, which slows time-to-value during initial setup and can create a negative experience for first-time users, to the extent that we took our demo Render instances offline.

Solution

Add a set of simple, dismissible onboarding cards to the Fleet UI (probably dashboard to start with – we could use the existing "Learn how to use Fleet" card.)

This feature would go a long way toward strengthening the case for reintroducing our demo environment in a way that's positive for both new users and Fleet.

See this ticket for wireframe concepts that are very close to ready:
#37971

Screenshots and/or screen recording

[noahtalerman]

Thanks @mike-j-thomas! I queued this one up for #g-first-impressions.