docs(user): add a model-provider configuration guide

The guide tier said how to compose plugins with `cordis.yml` but never how to
reach a provider other than DeepSeek, so the two things a person actually does
— give a catalog provider its key from the Models page, and declare a gateway
the installed catalog does not ship — had no home outside package READMEs.

The new page covers both entry points and the relationship between them: the
Models page and `$DSH_HOME/settings.yaml` write one document, over a
`llm-pi-ai` adapter that mounts dormant until that document names routes. It
carries the settings shape, catalog replacement and its capacity fallbacks,
credential references, and the four failures a misconfigured route produces,
and links the generated config catalog for exhaustive fields.

It sits between Quick start and Configuration in the guide sidebar, which is
where a reader hits the question.

docs/user/guide/quickstart.md

@@ -57,5 +57,6 @@ headless-agent uses the `@deepseek-ai/dsh-cli-demo` app. `dsh web` instead compo
 
 ## Next steps
 
+- [Model providers](./providers.md) — reach providers beyond DeepSeek, and custom gateways
 - [Configuration](./config.md) — understand the `cordis.yml` format
 - [Develop a plugin](../develop/basic/) — build your own tool or backend