Add a markdownlint rule for code blocks

This sets a specific rule for MD046 (code block style) instead of the default
value of "consistent".

.mdl_config.yaml

@@ -43,3 +43,8 @@ MD033:
 MD035:
   # Enforce dashes for horizontal rules
   style: "---"
+
+# MD046/code-block-style Code block style
+MD046:
+  # Enforce the fenced style for code blocks
+  style: "fenced"