Install step does not include `linkify` dependency

[jhcole]

Describe the gap in the current documentation

Following the current contribution guidelines, some tests fail when running pytest if linkify is not installed.

========================================================== short test summary info ===========================================================
FAILED tests/test_renderers/test_myst_config.py::test_cmdline[40-linkify] - SystemExit: 1
FAILED tests/test_renderers/test_myst_config.py::test_cmdline[50-gfm-strikethrough] - SystemExit: 1
FAILED tests/test_renderers/test_myst_config.py::test_cmdline[68-gfm-disallowed-html] - SystemExit: 1
FAILED tests/test_renderers/test_myst_config.py::test_cmdline[95-gfm-autolink] - SystemExit: 1
FAILED tests/test_sphinx/test_sphinx_builds.py::test_extended_syntaxes - ModuleNotFoundError: Linkify enabled but not installed.
======================================== 5 failed, 1057 passed, 12 skipped, 29923 warnings in 16.45s =========================================

Describe the solution you'd like

Add linkify as an optional dependency in the pip install step.