docs: move plugin factory docs to guide troubleshooting page

Address review feedback from @cpojer:
- Merge into the single existing troubleshooting page instead of
  adding a separate config troubleshooting page
- Remove the VP_COMMAND implementation detail note (not useful to users)

Author: fengmk2

docs/.vitepress/config.mts

@@ -166,7 +166,6 @@ export default extendConfig(
                 { text: 'Build', link: '/config/build' },
                 { text: 'Pack', link: '/config/pack' },
                 { text: 'Staged', link: '/config/staged' },
-                { text: 'Troubleshooting', link: '/config/troubleshooting' },
               ],
             },
           ],

docs/config/troubleshooting.md

@@ -89,6 +89,37 @@ export default defineConfig({
 });
 ```
 
+## Slow config loading caused by heavy plugins
+
+When `vite.config.ts` imports heavy plugins at the top level, every `import` is evaluated eagerly, even for commands like `vp lint` or `vp fmt` that don't need those plugins. This can make config loading noticeably slow.
+
+Pass a factory function to `plugins` in `defineConfig` to defer plugin loading. The factory is only called for commands that need plugins (`dev`, `build`, `test`, `preview`), and skipped for everything else:
+
+```ts
+import { defineConfig } from 'vite-plus';
+
+export default defineConfig({
+  plugins: () => [myPlugin()],
+});
+```
+
+For heavy plugins that should be lazily imported, combine with dynamic `import()`:
+
+```ts
+import { defineConfig } from 'vite-plus';
+
+export default defineConfig({
+  plugins: async () => {
+    const { default: heavyPlugin } = await import('vite-plugin-heavy');
+    return [heavyPlugin()];
+  },
+});
+```
+
+::: warning
+The plugins factory requires `defineConfig` from `vite-plus`, not from `vite`. Vite's native `defineConfig` does not support factory functions for the `plugins` field.
+:::
+
 ## Asking for Help
 
 If you are stuck, please reach out: