Fix some broken fragment links

[StevenMaude]

Partially deals with #1500.

This PR fixes the fragment links that we can fix.

Of those that remain, there are three classes that are all probably limitations of how lychee detects fragments:

• most links are in ehrQL's autogenerated documentation
  • from a quick look at one or two, these are probably OK, but there is something blocking correct autodetection of the links (see Check anchor links #1500 for more)
• the link to opensafely-cli.md#jupyter-running-jupyterlab is correct but not correctly detected
  • likely because of how lychee generates the expected headings and there being backtick formatting in this heading
• the link to study-def-variables.md#cohortextractor.patients.satisfying is correct but not correctly detected
  • because the content is added in by mkdocstrings at build-time and the link is not in the Markdown source

Note that this means we can't easily add in the --include-fragments option to lychee, unless we manually exclude these links. Otherwise the check will always fail.

[StevenMaude]

For reference, these are the failures that remain.

Errors per input
Errors in docs/ehrql/how-to/dummy-measures-data.md

    [ERR] file:///home/runner/work/documentation/documentation/docs/ehrql/how-to/dummy-data.md#supply-your-own-dummy-data | Failed: Cannot find fragment

Errors in docs/ehrql/explanation/measures.md

    [ERR] file:///home/runner/work/documentation/documentation/docs/ehrql/reference/language.md#Measures.define_defaults | Failed: Cannot find fragment
    [ERR] file:///home/runner/work/documentation/documentation/docs/ehrql/reference/language.md#BoolPatientSeries | Failed: Cannot find fragment
    [ERR] file:///home/runner/work/documentation/documentation/docs/ehrql/reference/language.md#weeks.starting_on | Failed: Cannot find fragment
    [ERR] file:///home/runner/work/documentation/documentation/docs/ehrql/reference/language.md#months.starting_on | Failed: Cannot find fragment
    [ERR] file:///home/runner/work/documentation/documentation/docs/ehrql/reference/language.md#Measures.configure_dummy_data | Failed: Cannot find fragment
    [ERR] file:///home/runner/work/documentation/documentation/docs/ehrql/reference/cli.md#generate-measures | Failed: Cannot find fragment
    [ERR] file:///home/runner/work/documentation/documentation/docs/ehrql/reference/language.md#Measures.define_measure | Failed: Cannot find fragment
    [ERR] file:///home/runner/work/documentation/documentation/docs/ehrql/reference/language.md#years.starting_on | Failed: Cannot find fragment

Errors in docs/reports/intro.md

    [ERR] file:///home/runner/work/documentation/documentation/docs/opensafely-cli.md#jupyter-running-jupyterlab | Failed: Cannot find fragment

Errors in docs/study-def.md

    [ERR] file:///home/runner/work/documentation/documentation/docs/study-def-variables.md#cohortextractor.patients.satisfying | Failed: Cannot find fragment

Errors in docs/ehrql/how-to/define-population.md

    [ERR] file:///home/runner/work/documentation/documentation/docs/ehrql/reference/language.md#BoolPatientSeries.and | Failed: Cannot find fragment

Errors in docs/ehrql/reference/language.md

    [ERR] file:///home/runner/work/documentation/documentation/docs/ehrql/reference/schemas/core.md#patients | Failed: Cannot find fragment

Errors in docs/ehrql/how-to/test-dataset-definition.md

    [ERR] file:///home/runner/work/documentation/documentation/docs/ehrql/reference/cli.md#assure | Failed: Cannot find fragment
    [ERR] file:///home/runner/work/documentation/documentation/docs/ehrql/reference/schemas/core.md#patients | Failed: Cannot find fragment
    [ERR] file:///home/runner/work/documentation/documentation/docs/ehrql/reference/schemas/core.md#medications | Failed: Cannot find fragment

[LFISHER7]

Thanks for fixing these @StevenMaude!