Improve XMTP agent getting started guide with logical flow

[wamimi]

What changed? Why?

This PR improves the Getting started with Chat Agents guide by restructuring it with a clear, logical flow that prevents confusion.

The Problem

The current documentation jumps between concepts without a clear workflow:

• Shows "Installation" but doesn’t explain where to install
• Shows "Usage" code but doesn’t explain where to put it
• Tells users to create .env manually but doesn’t explain how to get the values (XMTP_WALLET_KEY, XMTP_DB_ENCRYPTION_KEY)
• Missing the critical import dotenv/config, which causes XMTP_WALLET_KEY format errors

The Solution

Restructured the guide with a clear 6-step workflow:

1. Clone the XMTP agent examples repository — Gives users a workspace and installs all dependencies
2. Generate keys with npm run gen:keys — Automatically creates .env with all required values
3. Create index.ts file — Clear instruction on where to put the code
4. Add agent code — Includes the critical import dotenv/config line
5. Run the agent — Use npx tsx --watch index.ts
6. Test on xmtp.chat — Step-by-step testing instructions

Notes to reviewers

The new flow follows the natural developer journey:
setup workspace → generate credentials → write code → run → test.
This eliminates the confusion of “where do I put this?” and “how do I get these values?” that plagued the previous version.

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How has it been tested?

Spun up documentation locally with mint dev
Tested locally and amde sure the agents runs and responds with Hello from my Base App Agent! 👋

[cb-heimdall]

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