fix heading and install markdownlint if needed

docs/Building_Diagrams/Extensions.md

@@ -83,7 +83,7 @@ An entirely new feature application feature with frontend and backend components
 
 ![Extension](images/Agregate_metadata.png)
 
-# Use Cases
+## Use Cases
 
 If your organization has specific needs not catered to by the standard SpiffArena features, you can use extensions to add those features.  
 

docs/Makefile

@@ -13,7 +13,7 @@ help:
 	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O) && echo "  \033[0;34mlint\033[0m        runs markdownlint on all markdown files (this was added to the Makefile manually. pardon formatting)"
 
 lint:
-	markdownlint **/*.md
+	(markdownlint -V > /dev/null || npm install -g markdownlint-cli) && markdownlint **/*.md
 
 .PHONY: help Makefile