Update job writing docs

[mtuchi]

Description
Improvements for job writing guide docs.

• Update Referencing credential secrets in your job code

We don't want to encourage developers to write credentials in job code, Authentication should be handled by the adaptor it self. The current content is bad practices

• Add a debugging section

### Debugging

- Use `console.log()` liberally during development
- Test with small data sets first
- Use the OpenFn CLI to test locally
- Check compiled code with `openfn compile` if needed

[mtuchi]

Should we add this ? 🤔

## Common Pitfalls to Avoid

1. **Nested Operations** - Always keep operations at top level
2. **Forgetting to Return State** - Every callback must return state
3. **Reading State Too Early** - Use `$` or arrow functions
4. **Not Handling Errors** - Add error handling for production code
5. **Bloated Final State** - Clean up state before job completion
6. **Hardcoded Values** - Use configuration or state for dynamic values
7. **Complex Callbacks** - Break complex logic into multiple operations