Add user guide and adoption-helpful README signposts

[iainmcgin]

Closes #47

Adds `docs/guide.md` as the long-form companion to the crate README, plus README updates that surface the guide and the examples directory near the top.

Stacked on #46 - this PR's base is `feat/examples-streaming-middleware` because the guide and README reference `examples/streaming-tour/` and `examples/middleware/` from #46. Once #46 merges, GitHub will rebase this onto main automatically (or I'll do it manually).

docs/guide.md

801 lines covering:

Section	What it covers
Installation	Crates, MSRV (1.88), feature flags, common combinations
Quick start	One runnable unary example end-to-end
Code generation	Both build paths (`connectrpc-build` for build-time, `buf generate` for checked-in code)
Implementing servers	Handler signatures, the `Context` parameter (headers/trailers/extensions/deadline/compression), `ConnectError` returns, `Router` composition
Streaming RPCs	Server / client / bidi handler signatures with snippets, plus client-side calling patterns
Tower middleware	`ServiceBuilder` composition mounted on `axum::Router::layer()`, identity passthrough from layer extensions to `Context::extensions`, short-circuit responses
Hosting	axum (recommended), standalone `Server`, TLS for both
Clients	`HttpClient` plaintext + TLS, `ClientConfig` defaults, `CallOptions` per-call overrides, three response access patterns (`view()`, `into_view()`, `into_owned()`), custom transports
Errors and status codes	`ConnectError` shape, `ErrorCode` set, automatic protocol-specific serialization
Compression	Per-RPC override via `Context::set_compression`, custom `CompressionProvider` registration
Examples directory tour	One-line summary of every example crate

The guide references the streaming-tour and middleware examples (PR #46) for runnable patterns rather than duplicating handler code inline. Each section is self-contained enough to land via search or external link.

README updates

• New Status / MSRV / Documentation block right after the tagline, with pointers to the guide, examples directory, and docs.rs.
• New Streaming, middleware, TLS subsection at the end of Quick Start with focused links into specific guide sections and example crates.

19 line diff to README; everything else stays as it was.

Merge dependencies

• Pin MSRV to Rust 1.88 and verify in CI #44 (MSRV pin) sets `rust-version = "1.88"` which the guide references. This PR's branch is off main and doesn't reflect the MSRV pin yet, so the workspace inheritance won't propagate to the new examples. Either Pin MSRV to Rust 1.88 and verify in CI #44 lands first and this branch gets rebased to pick up the workspace inheritance, or it lands after and a tiny follow-up adds `rust-version.workspace = true` to the new example Cargo.tomls.
• Add streaming-tour and middleware examples #46 (examples) is required by this PR (stacked on it). GitHub will not let this merge until Add streaming-tour and middleware examples #46 merges first.

[asacamano]

This is so awesome! Just reading over it I was think "yes, this is exactly what it should be like."