We don't have documentation for this still - which considering our entire infrastructure is designed to run MediaWiki, it's amazing we've gone 5 years without documenting it.
While not the ideal and perfectly complete documentation, it's there now. There can obviously be more details, improvements, etc. but I think what I've done is satisfactory for this task to be deemed resolved. https://meta.miraheze.org/wiki/Tech:MediaWiki_appserver