docs: broaden README with event sourcing and CQRS positioning and ecosystem links

[woksin]

What changed

README-only changes to make the samples repository easier to discover and to reflect the breadth of the Cratis stack accurately:

• Tagline now leads with the search terms people actually use: "Runnable event sourcing and CQRS samples for the Cratis stack," naming the Chronicle event store, Arc CQRS for ASP.NET Core, React Components, the CLI, Workbench, and the model-first Screenplay/Stage (experimental).
• "What the journey looks like" now links each learning path to its destination: Arc documentation, Chronicle documentation, and the Screenplay/Stage repositories.
• New "The wider Cratis ecosystem" section cross-links the surrounding projects — Chronicle and its language-agnostic gRPC clients (.NET, TypeScript, Kotlin/Java, Elixir; Python coming soon), Arc, Components, CLI + Workbench, the experimental model-first layer, and supporting tools — and states plainly that everything Cratis publishes today is MIT licensed and free to use.

Why

The samples repo is a common entry point for people evaluating the stack, but the README did not mention "event sourcing" or "CQRS" in its tagline and had no outbound links to the rest of the ecosystem. This aligns it with the positioning used across the other Cratis repositories.

Suggestion (not in this PR)

samples.json is intentionally untouched. As a follow-up, consider adding sample tracks for the non-.NET Chronicle clients (TypeScript, Kotlin/Java, Elixir) so cratis.io/samples reflects the multi-language breadth — best planned through a dedicated issue.