Add manual

1. Why is this change necessary?

2. What does this change do, exactly?

3. Describe each step to reproduce the issue or behavior.

4. Please link to the relevant issues (if any).

5. Checklist

• I have updated the plugin version and changelog
• I have updated the manual / created a manual update merge request
• I have updated the shopware store information
• I can confirm the code style fits our requirements
• I can confirm that no debug output is generated anywhere

6. Release Notes