Document that deployment steps are just Python functions

[zzstoatzz]

Summary

Users often don't realize that deployment steps in prefect.yaml are just fully qualified function names that they can write themselves. This creates confusion when users need custom functionality (like loading environment variables from a file) and can't find a built-in step.

Current State

• The documentation doesn't clearly explain that steps are just Python functions
• Users assume they can only use built-in steps
• Custom steps are possible but not well documented

Proposed Documentation

Should clearly document:

1. Steps are just fully qualified Python function names
2. How to write custom step functions
3. Example of a custom (e.g. load_dotenv) step
4. How to reference custom steps in prefect.yaml

Examples in the Wild

• Custom step example: https://github.com/zzstoatzz/prefect-pack/blob/main/src/prefect_pack/steps/events.py#L37
• Usage in prefect.yaml: https://github.com/zzstoatzz/prefect-pack/blob/main/prefect.yaml#L46

User Impact

This documentation would help users:

• Understand they can extend deployment functionality
• Create custom steps for their specific needs