Add user guide and adoption-helpful README signposts

[iainmcgin]

The README is detailed about API surface and performance, but new adopters miss several signals:

• No long-form documentation beyond the README and rustdoc, so deeper questions ("how do tower layers compose with the connect router?", "how do I read identity from a custom auth layer in my handler?") require reading source.
• No MSRV declared anywhere outside the Cargo manifest.
• No stability statement (pre-1.0, API surface settling).
• No top-of-README pointer to the examples/ directory.

Plan

Add docs/guide.md as the long-form companion, covering:

1. Installation (deps, MSRV, feature flags)
2. Quick start
3. Code generation (both build paths)
4. Implementing servers (handler signatures, Context, errors, headers/trailers, registering)
5. Streaming RPCs (all 4 types)
6. Tower middleware (layer composition, identity passthrough via Context::extensions)
7. Hosting (axum, standalone, TLS)
8. Clients (HttpClient, ClientConfig, CallOptions, response access patterns, custom transports)
9. Errors and status codes
10. Compression
11. Examples directory tour

The guide references the streaming-tour and middleware examples (added in #45 / PR #46) for runnable patterns rather than duplicating those examples inline.

README updates: add a Status / MSRV / Documentation block near the top with pointers to the guide, examples, and docs.rs. Add a focused "Streaming, middleware, TLS" subsection at the end of Quick Start linking into specific guide sections + examples.