Add ehrQL placeholders

[milanwiedemann]

This adds a placeholer ehrql dataset definition and action with instructions for cohortextractor and ehrql users at the top of each script (dataset_definition.py and study_definition.py).

This will be helpful for creating ehrQL documentation and examples because if gives us a starting point that is easy for users to get to. It's also a good to introduce ehrql in the research template simply because we are moving away from cohortextractor.

The dataset_definition.py is not a direct translation of the study_definition.py because I think it's good to use an ehrql method (age_on) in the example and show how a population can be defined using two conditions.

However, we should have think if this is a good starting point for all studies that use the research-template or whether we want to make it more simple/complex?

[inglesp]

However, we should have think if this is a good starting point for all studies that use the research-template or whether we want to make it more simple/complex?

Maybe? But we don't need to decide that now.

Do we need to update anything in the docs?

[milanwiedemann]

Maybe? But we don't need to decide that now.

Agree!

Do we need to update anything in the docs?

At this stage I don't think we have to change anything. I looked at the docs and the research template gets a lot of mentions but because we didn't change the default behaviour I think we can merge this without making changes to the docs!