Add site_url to fix issue where mkdocs would automatically set use_directory_urls to false.

mkdocs.yml

 site_name: STACK Docs
+site_url: https://docs.stack-assessment.org/
 extra_javascript: 
     - 'https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/MathJax.js?config=TeX-AMS-MML_HTMLorMML'