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.

website/docs.ts

@@ -130,13 +130,21 @@ const homeAndGuide = pairedPages([
     section: { root: '入门', en: 'Guide' },
     order: 2,
   },
+  {
+    source: 'docs/user/guide/providers.md',
+    route: 'guide/providers.md',
+    label: { root: '配置模型提供方', en: 'Model providers' },
+    sidebar: { root: 'zh-guide', en: 'en-guide' },
+    section: { root: '入门', en: 'Guide' },
+    order: 3,
+  },
   {
     source: 'docs/user/guide/config.md',
     route: 'guide/config.md',
     label: { root: '配置文件', en: 'Configuration' },
     sidebar: { root: 'zh-guide', en: 'en-guide' },
     section: { root: '入门', en: 'Guide' },
-    order: 3,
+    order: 4,
   },
 ])