feat(docs): add hint about nx add

Author: michaelbe812

apps/docs/src/content/docs/getting-started/installation.md

@@ -20,6 +20,11 @@ The recommended approach is to use the new modular package structure with the co
 
 ### Step 1: Install the Core Package
 
+```bash
+nx add @nx-plugin-openapi/core 
+```
+
+or 
 ```bash
 npm install --save-dev @nx-plugin-openapi/core
 ```