Test ehrQL dataset definitions that are outside of the core ehrQL documentation

[StevenMaude]

We now test dataset definitions in the ehrQL documentation that are:

• code blocks labelled with ehrql in the ehrQL repository's Markdown source files inside docs/
• Python files in the ehrQL repository's docs/ directory

We don't do this in the rest of the documentation. For example, the Getting Started guide now has an ehrQL dataset definition which is not regularly tested.

This is fine for now, because there are not many dataset definitions. But we may wish to in future, particularly if there are a few of these.

The same code could probably be used as in opensafely-core/ehrql#1648, ideally reusing the test code in-place and running against this repository's docs/ directory.