Enable markdownlint rule MD040

Markdownlint rule MD040 requires all code blocks to declare a
language. This MR should only be merged after all code blocks
have been proven to be fixed.

.markdownlint.json

@@ -22,7 +22,6 @@
     "style": "---"
   },
   "no-emphasis-as-heading": false,
-  "fenced-code-language": false,
   "first-line-h1": false,
   "code-block-style": {
     "style": "fenced"