rustdoc: Enable Markdown extensions when looking for doctests

We should enable these to avoid misinterpreting uses of the extended
syntax as code blocks. This happens in practice with multi-paragraph
footnotes, as discovered in #139064.

tests/rustdoc-ui/multi-par-footnote.rs

@@ -0,0 +1,18 @@
+//@ check-pass
+//@ compile-flags:--test
+//@ normalize-stdout: "finished in \d+\.\d+s" -> "finished in $$TIME"
+// Regression test for #139064.
+
+/// Example
+///
+/// Footnote with multiple paragraphs[^multiple]
+///
+/// [^multiple]:
+///     One
+///
+///     Two
+///
+///     Three
+pub fn add(left: u64, right: u64) -> u64 {
+    left + right
+}