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.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write docs/user/guide/quickstart.md
-quickstart.md: 199b3f092159fa6fbaf3ae298151487c924ac6f1
-quickstart.zh.md: 9327ed646ba211bcce6426beb6bf76fca50acbf6
+quickstart.md: 8a9ed716d9395448aadfb97d0935bd42ee06e6c1
+quickstart.zh.md: 3652b0453f870640b278ce6f1355e67e85983ffe