Added dashboard & visualization docs to readme

[ivpusic]

We recently added visualization and dashboard support to the CLI, updating README to reflect that.

[cursor]

PR Summary

Low Risk
Documentation-only changes with no impact on runtime behavior.

Overview
Updates README.md to reflect new CLI capabilities by expanding the intro to mention visualization and dashboard management.

Adds documentation for the new dune visualization (viz) and dune dashboard (dash) command groups, including supported subcommands/flags and visualization types.

^{Reviewed by Cursor Bugbot for commit 4a64a6f. Configure here.}