fix: update linter for new challenge format (#40317)

Also uses yaml for better vscode integration

curriculum/challenges/.markdownlint.yaml

@@ -0,0 +1,10 @@
+default: true  # include all rules, with exceptions below
+MD002: false   # first heading should not be a top level heading
+MD013: false   # lines can be any length
+MD022: false   # headings don't need surrounding by newlines
+MD025: false   # headings are used as markers by the parser
+MD031: true    # fenced blocks do need surrounding by newlines
+MD033: false   # inline html is required
+MD040: false   # TODO: enable this when the challenges have been fixed
+MD036: false   # TODO: **Example** is the main offender, should that be a heading?
+whitespace: false  # extra whitespace is ignored, so we don't enforce it.