docs: add comprehensive testing and Discord bot setup guide

Co-authored-by: aider (openai/unsloth/Qwen3-Coder-Next) <aider@aider.chat>

!receipts

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+
+## Troubleshooting
+
+### Bot doesn't respond to commands
+- Make sure MESSAGE CONTENT INTENT is enabled in the Discord Developer Portal
+- Check that the bot has the correct permissions in the server
+
+### PDF parsing fails
+- Ensure the PDF is text-based (not scanned images)
+- Check the PDF file is not corrupted
+- Verify pdfplumber is installed correctly
+
+### Database errors
+- Delete the `grocery_receipts.db` file to reset the database
+- The database will be recreated automatically on next run
+
+### Bot can't connect to Discord
+- Verify your bot token is correct
+- Check your internet connection
+- Ensure the bot has been added to your server with correct permissions
+
+## Expected Output
+
+When a PDF receipt is successfully processed, you should see:
+- A confirmation message in Discord: "Receipt 'filename.pdf' processed! Found X items."
+- A new entry in the `receipts/` folder
+- New entries in the database (visible via `!receipts` command)
+
+## Next Steps
+
+Once the basic functionality is working:
+- Try uploading different receipt formats
+- Test with various store names and product combinations
+- Experiment with the price comparison features