docs(readme): beginner-first rewrite + redesigned dashboard screenshot

Rewrite the README around a fast first-run path (install → nexus dev → a verified first-handler example that serves GraphQL + REST + the dashboard), with deep topics moved to a compact "going further" links table. Fixes two latent bugs from the old quick start: Params field p.Input → p.Args, and the missing Introspection: true needed to open /__nexus under plain `go run`. Corrects nexus.AsCall → peer.AsCall.

Regenerate docs/dashboard.png from the redesigned dashboard (module selected, showing the live Inspector facets). Remove three orphaned dashboard components (ActivityRail, OverlayToggles, TimeScrubber) left unused after the redesign.

Author: Manpaul1