Add Providers docs page

Splits first-class and custom provider docs out of the Configuration
page onto /docs/providers. Introduces the first-class providers
(claude, codex, opencode, copilot, pi) and covers extending a provider,
Z.AI and Qwen plans, multiple profiles, custom binaries, ACP agents,
additionalModels, and disabling. Points to docs/CUSTOM-PROVIDERS.md on
GitHub as the source of truth.

packages/website/src/routes/docs.tsx

@@ -12,6 +12,7 @@ const navigation = [
   { name: "Git worktrees", href: "/docs/worktrees" },
   { name: "CLI", href: "/docs/cli" },
   { name: "Skills", href: "/docs/skills" },
+  { name: "Providers", href: "/docs/providers" },
   { name: "Configuration", href: "/docs/configuration" },
   { name: "Security", href: "/docs/security" },
   { name: "Best practices", href: "/docs/best-practices" },