docs: add pnpm instructions

Author: danielroe

docs/content/1.index.md

@@ -13,7 +13,7 @@ cta:
 secondary:
   - Star on GitHub →
   - https://github.com/nuxt/image
-snippet: yarn add -D @nuxt/image
+snippet: pnpm add -D @nuxt/image
 ---
 
 #top

docs/content/2.get-started.md

@@ -14,6 +14,9 @@ You are reading the `v1` documentation compatible with **Nuxt 3** (experimental)
 Add `@nuxt/image@rc` as a development dependency to your project:
 
 ::code-group
+```bash [pnpm]
+pnpm add -D @nuxt/image@rc
+```
 ```bash [yarn]
 yarn add --dev @nuxt/image@rc
 ```
@@ -57,6 +60,10 @@ If an error occurs during installation:
 - Try to upgrade to latest versions:
 
 ::code-group
+  ```bash [pnpm]
+  pnpm up @nuxt/image@rc
+  ```
+
   ```bash [yarn]
   yarn upgrade @nuxt/image@rc
   ```