docs: add credentials prerequisites to create-basic-agent guide (#11913)

## Summary
Addresses #11785 - users were encountering `openai_api_key_credentials`
errors when following the create-basic-agent guide because it didn't
mention the need to configure API credentials before using AI blocks.

## Changes
Added a **Prerequisites** section to
`docs/platform/create-basic-agent.md` explaining:
- **Cloud users:** Go to Profile → Integrations to add API keys
- **Self-hosted (Docker):** Add keys to `autogpt_platform/backend/.env`
and restart services

Also added a note that the Calculator example doesn't need credentials,
making it a good first test.

## Related
- Issue: #11785

docs/platform/create-basic-agent.md

@@ -4,6 +4,28 @@
 
 This guide walks through creating a simple question-answer AI agent using AutoGPT's visual builder. This is a basic example that can be expanded into more complex agents.
 
+## **Prerequisites**
+
+### **Cloud-Hosted AutoGPT**
+If you're using the cloud-hosted version at [agpt.co](https://agpt.co), you're ready to go! AI blocks come with **built-in credits** — no API keys required to get started. If you'd prefer to use your own API keys, you can add them via **Profile → Integrations**.
+
+### **Self-Hosted (Docker)**
+If you're running AutoGPT locally with Docker, you'll need to add your own API keys to `autogpt_platform/backend/.env`:
+
+```bash
+# Create or edit backend/.env
+OPENAI_API_KEY=sk-your-key-here
+ANTHROPIC_API_KEY=sk-ant-your-key-here
+# Add other provider keys as needed
+```
+
+After adding keys, restart the services:
+```bash
+docker compose down && docker compose up -d
+```
+
+**Note:** The Calculator example below doesn't require any API credentials — it's a good way to test your setup before adding AI blocks.
+
 ## **Example Agent: Q&A (with AI)**
 
 A step-by-step guide to creating a simple Q&A agent using input and output blocks.