docs: add testing, asyncapi, and DI example blocks to the README

[powersemmi]

Description

Expand the README with three example blocks so a reader sees the common flows without leaving the page:

• Testing the service (was the one-line Test it) - drive a built app with the TestApp harness against the in-memory broker, inject input, and assert handler settlement plus a downstream publish.
• Project documentation (was the one-line Documentation) - build the AsyncAPI spec and the interactive viewer HTML programmatically with build_spec / render_viewer_html. The CLI ruststream asyncapi gen already appears under Run it, so this covers the in-process path.
• Injecting dependencies (new) - declare app state, derive FromRef, and take a dependency as a State<T> handler argument instead of reaching through ctx.state().

Each block links to its compiling examples/*.rs source of truth (examples/testing.rs, examples/asyncapi_http.rs, examples/from_context.rs) so the snippets cannot silently drift; GitHub-rendered README does not support file includes, so the snippets are inline fences trimmed from those examples.

Also drop the stale ruststream-py Ecosystem line: Python bindings were removed, the reference no longer points at a maintained crate.

Fixes #130

Type of change

• Documentation (typos, code examples, or any documentation updates)
• Bug fix (a non-breaking change that resolves an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (removing or renaming public API, changing a signature, tightening bounds, or
  raising the MSRV - a minor version bump pre-1.0)
• This change requires a documentation update

Checklist

• My code follows the project's style guidelines (just check passes: rustfmt, clippy, and
  cargo check with all features and with --no-default-features)
• I have performed a self-review of my own code
• I have made the necessary changes to the documentation (rustdoc and/or the docs site)
• My changes generate no new warnings (clippy runs with -D warnings)
• I have added tests that validate my fix or new feature
• New and existing tests pass locally (just test)
• Public items carry a compiling # Examples doctest, and user-facing changes are reflected in
  examples/ where applicable