chore: install package for building documentation

Installation is required for cross-references.

Author: Midnighter

tox.ini

@@ -57,7 +57,6 @@ commands=
     mypy {toxinidir}/src/structurizr {toxinidir}/examples
 
 [testenv:docs]
-skip_install = True
 deps=
     -r{toxinidir}/docs/requirements.txt
 commands=