Add markdown linting (without breaking the site this time!)

.markdownlint.yaml

@@ -0,0 +1,17 @@
+# What's this for? This file is used by the markdownlinting extension in VSCode, as well as the GitHub actions
+# See all rules at https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md
+
+# Ignore line length
+"MD013": false
+
+# Allow multiple headings with the same content provided the headings are not "siblings"
+"MD024":
+    "siblings_only": true
+
+# Allow trailing punctuation in headings
+"MD026": false
+
+# We use fenced code blocks, but this test conflicts with the admonitions plugin we use, which relies 
+# on indentation (which is then falsely detected as a code block)
+"MD046": false
+