docs(readme): rebase PR #420 onto main and polish for CLI zero-arg flow

Squashes the README overhaul (#415) and follow-up polish (#420) onto current main
and updates the example flows for the unified CLI introduced in #431.

- Adopt zero-arg defaults: prove/verify run with no positional args from the
  example directory; prepare invokes nargo internally so the explicit
  nargo compile step is gone for the basic flow.
- Fix CLI badge to point to ci.yml (the actual workflow).
- Fix license declaration to MIT only (License.md and Cargo.toml are MIT,
  not MIT/Apache-2.0 as PR #420 claimed).
- Fix profiling docs: feature is profiling-allocator (default-on), tracy
  needs --features tracy plus the --tracy runtime flag.
- Replace the architecture mermaid with one that shows the actual data flow
  (.pkp/.pkv files, Prover.toml input, generate-gnark-inputs handoff).
- Tighten Acknowledgements with concrete crate-level integration points.

Banner asset (assets/banner.png) carried over from #415.

Author: Bisht13