b314

docs: add "Why llmman?" section and move metrics reference (#402)

The README opened with what llmman does but never said why someone
would pick it over Ollama, the tool most readers already have. Add a
short section right after the intro that states the four claims that
matter: provider agnostic, registry agnostic, vanilla llama.cpp/GGUF/
safetensors with no private blob format, and one-step transfer from
HuggingFace into your own registry for air-gapped or compliance-bound
setups. A table lays the same points out side by side with Ollama.

The README also carried the full Prometheus metric table, scrape config
and PromQL hints inline under "Serve", which is operator reference
material rather than something a first-time reader needs. Move it to
docs/metrics.md next to configuration.md and verification.md, and leave
a short pointer behind. Content is unchanged apart from headings.