Case control and measures pages haven't been updated to use ehrQL

[StevenMaude]

Examples:

documentation/docs/case-control-studies.md

Lines 157 to 168 in b635d4f

	Our *project.yaml* now includes the following action:
	```yaml
	# ...
	actions:
	# ...
	extract_controls:
	run: cohortextractor:latest generate_cohort --study-definition study_definition_controls --output-format csv.gz
	needs: [matching]
	outputs:
	highly_sensitive:
	cohort: output/input_controls.csv.gz

documentation/docs/measures.md

Lines 21 to 34 in b635d4f

	### Define a study definition
	The `study_definition.py` script for the example above is:
	```py
	from cohortextractor import StudyDefinition, Measure, patients
	study = StudyDefinition(
	# Configure the expectations framework
	default_expectations={
	"date": {"earliest": "2020-01-01", "latest": "today"},
	"rate": "exponential_increase",
	"incidence": 0.2,
	},

[lucyb]

I've spoken to Brian and someone in the research team will be able to update this code.

[wjchulme]

Regarding measures. There are currently two pages covering measures in the documentation.

• the new measures in ehrQL page, filed under ehrQL: https://docs.opensafely.org/ehrql/explanation/measures/
• the old measures in cohort extractor page, filed under "legacy": https://docs.opensafely.org/measures/

Anything under legacy doesn't show up in searches. So the page is harmless and can stay there. As far as I can see there's nothing that needs to be updated for the new measures ehrQL page, so we're done. I don't know which commit(s) fixed this between this issue being opened and now, so I won't link to it.

All that remains for this issue is the case-control docs, which @alexwalkerepi has agreed to take a look at.