Move docs for user-defined rules to a separate page

[deemp]

Details

Currently, user-defined rules are documented on the normalizer transform page (link).

However, these rules are used in the normalizer dataize command too.

Hence, we need to provide a comprehensive documentation for user-defined rules on a separate page.

Subtasks

• [ ] Describe the YAML format
• [ ] Explain how to write rules
• [ ] Explain conditions