docs(readme): rewrite for the 0.5 release

Lead with the three big themes accumulated since 0.4.0:

  - Axum server codegen (P1-P5: `[server]` TOML section, `server
    list/add/remove` CLI, trait-per-tag + response enum + router
    factory, SSE-aware `OkStream` variant, required-param 400,
    multi-tag `build_router`, opt-in model pruning).
  - OpenAPI 3.1 + 3.2 conformance (38 of 51 beads from #14: strict
    Extensions, version gate, full Components family, JSON Schema
    2020-12 keywords, 3.2 deltas, webhooks ingest, dynamic refs).
  - Real-world spec compile guarantee — 54 specs now compile cleanly
    (was 20), guarded by CI.

Restructured into:

  - "What's new in 0.5" up top (the elevator pitch).
  - Quick-start sections split into "client" and "Axum server".
  - New "OpenAPI 3.1 / 3.2 support" section with status tables.
  - New "CLI" section documenting `server list/add/remove`,
    `--types-conservative`, `--regenerate`.
  - `[server]` block + `schema_extensions` added to the TOML reference.
  - `server/{mod,api,errors,router}.rs` added to the Generated Output
    table.
  - Per-area Release notes appended at the bottom.

Install snippet bumped 0.4 -> 0.5 (Cargo.toml bump comes with the
release cut).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lightsofapollo