Add FAQ section to Readme and Troubleshooting Tips to setup GCP Vertex (#1158)

Added a “Troubleshooting & Tips” section to the GCP Vertex documentation.

This section provides guidance for self-hosted users on common issues
they may encounter when setting up Google Vertex AI integration in Khoj.
Topics covered include permissions, region compatibility, prompt size
limits, API key testing, and secure key management with environment
variables. The goal is to improve the onboarding experience and reduce
setup errors for contributors and self-hosters using Vertex AI models
like Claude and Gemini.

Signed off by: brightally6@gmail.com

documentation/docs/advanced/gcp-vertex.md

@@ -24,3 +24,22 @@ Khoj can use Google's Gemini and Anthropic's Claude family of AI models from [Ve
    - **Max prompt size**: `60000` (replace with the max prompt size of your model)
    - **Tokenizer**: *Do not set*
 5. Select the chat model on [your settings page](http://localhost:42110/settings) and start a conversation.
+
+##  Troubleshooting & gcp AI Tips
+
+-  Permission Denied?  
+  Ensure your service account has the `Vertex AI User` role and that the API is enabled in your GCP project.
+
+-  Region Errors?  
+  Double-check that the model you're trying to use is supported in your selected region. Some Claude or Gemini models are restricted to specific zones like `us-east5` or `us-central1`.
+
+-  Prompt Size Limitations  
+  The "Max prompt size" should align with the limits defined in the model documentation. Exceeding it can silently fail or truncate inputs.
+
+-  Testing the API Key  
+  Before adding it to Khoj, you can verify that your key works by making a simple curl request to Vertex AI. This helps debug auth issues early.
+
+-  Use Environment Variables   
+  For better security, consider using environment variables to manage sensitive keys and inject them at runtime during base64 encoding.
+
+If you encounter any issues, the [Khoj Discord](https://discord.gg/BDgyabRM6e) is a great place to ask for help!