feat: check for trailing spaces in CI

test_trailing_spaces.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+
+TRAILING_SPACES=$(find content -name '*.md' -exec grep -EHn ' +$' {} \;)
+
+if [[ -n "$TRAILING_SPACES" ]]; then
+    echo "Error: these files have trailing spaces!"
+    echo "$TRAILING_SPACES"
+    exit 1
+fi