feat(genapi): add troubleshooting section for forbidden error

Add troubleshooting section for forbidden error

Author: fpagny

pages/generative-apis/troubleshooting/fixing-common-issues.mdx

@@ -13,6 +13,20 @@ dates:
 
 Below are common issues that you may encounter when using Generative APIs, their causes, and recommended solutions.
 
+## 403: Forbidden - Insufficient permissions to access the resource
+
+### Cause
+- You are not providing valid credentials
+- You do not have the required IAM permissions sets
+- You are not connecting to the right endpoint URL
+
+### Solution
+- Ensure you provide an [IAM Secret Key](https://www.scaleway.com/en/docs/iam/how-to/create-api-keys/) in your API request or third party library configuration
+- Ensure the [IAM user or application](https://www.scaleway.com/en/docs/iam/reference-content/users-groups-and-applications/) you are connecting with has the right [IAM permissions sets](https://www.scaleway.com/en/docs/iam/reference-content/permission-sets/) (either `GenerativeApisFullAccess` or a narrower one)
+- If you have access only to a specific project, ensure you are connecting to this project URL:
+  - The URL format is: `https://api.scaleway.ai/{project_id}/v1"`
+  - If no `project_id` is specified in the URL (`https://api.scaleway.ai/v1"`), your `default` project will be used.
+
 ## 429: Too Many Requests - You exceeded your current quota of requests/tokens per minute
 
 ### Cause