Add more jsonschema pre-commit hooks

[AlexWaygood]

check-readthedocs reports no errors but check-jsonschema complains:

Schema validation errors were encountered.
  .github/workflows/third_party.yml::$.jobs.skip-schedule-on-fork: {'name': 'Check for schedule trigger on fork', 'runs-on': 'ubuntu-latest', 'if': "github.repository == 'python/typing_extensions' || github.event_name != 'schedule'", 'steps': [{'run': True}]} is not valid under any of the given schemas
  Underlying errors caused this.

  Best Match:
    $.jobs.skip-schedule-on-fork: 'uses' is a required property
  Best Deep Match:
    $.jobs.skip-schedule-on-fork.steps[0].run: True is not of type 'string'

  1 other errors were produced. Use '--verbose' to see all errors.