M23: public documentation & examples review

[ausimian]

Summary

Pre-1.0 pass over the documentation surface users consume on HexDocs. No behavioural changes.

• Moduledoc pass. Added Public API sections to Emily, Emily.Backend, and Emily.Compiler distinguishing user-facing helpers from Nx.Backend / Nx.Defn.Compiler callback implementations. Added iex> examples to Emily.to_binary/1, shape/1, dtype/1, Emily.Compiler, Emily.Stream, Emily.Quantization, Emily.QuantizedWeight, Emily.MixedPrecision, and Emily.Telemetry. Reorganised Emily.Backend's divergences into a dedicated "Divergences from Nx.BinaryBackend" subsection.
• Zero-copy documentation. Emily.to_binary/1 now documents the buffer-refcount mechanism that blocks MLX's donation path and flags the BEAM ProcBin accounting footgun with three mitigations.
• HexDocs nav. mix.exs groups extras into README and Notebooks; modules into Core / Concurrency / Quantization / Training / Performance / Observability.
• README rewrite. Overview + Features bullets; Prerequisites section with the xcodebuild -downloadComponent MetalToolchain step; Building with a "Why vendored?" subsection covering Make CommandEncoder thread local ml-explore/mlx#3348 and #3405; expanded Concurrency model section covering the worker-thread architecture.
• Runnable Livebooks. notebooks/distilbert_qa.livemd (Bumblebee QA through Emily.Compiler) and notebooks/qwen3_quantized.livemd (Qwen3-0.6B int4-quantized + Emily.Stream concurrent serving), both self-contained via Mix.install/2.
• CHANGELOG cutover. Moved the M0–M22 release history from RELEASE.md into CHANGELOG.md, then collapsed it to a top-level feature summary under [0.1.0] - unreleased. Per-milestone detail now lives in the git history. RELEASE.md carries only M23 notes.
• mix docs runs clean (zero warnings).

Test plan

• mix precommit (4 doctests, 76 properties, 421 tests, 0 failures)
• mix test --only conformance (17 passing)
• mix docs (zero warnings)
• End-to-end build from a fresh clone in /tmp (git clone --recurse-submodules → mix deps.get → mix compile) with EMILY_CACHE overridden to force a cold MLX rebuild (cmake + Metal). README Usage example against the cold-built artefact produces [17.0, 39.0].