Add documentation for generating the vapid keys

issue #4 (closed)

content/en/docs/getting-started/generate-vapid-keys.md

+---
+title: "Generate VAPID-Keys"
+weight: 110
+toc: false
+---
+VAPID (Voluntary Application Server Identification) is a standard for secure web push notifications. It allows a web server to identify itself to a push service using public and private key pairs.  
+
+When you have NodeJS available in your path, running `npx web-push generate-vapid-keys --json` will output a json object containing a private key and a public key.  
+
+Now, given you are logged in to your Sharkey instance with the admin account,  
+you can enter those keys under Control Panel -> General. 
+Find the Section "ServiceWorker" and tick "Enable Push-Notifications for your browser",  
+it will prompt you for a private key and a public key.  
+You enter them, you're all set.  
+
+Have fun! 🥳