docs(test): add markdown renderer smoke test

doc/render-smoke.md

@@ -0,0 +1,11 @@
+# Markdown Render Smoke Test
+
+Inline math: $E = mc^2$.
+
+Display math:
+
+$$
+\int_0^1 x^2 \, dx = \frac{1}{3}
+$$
+
+If Markdown math rendering is enabled in Gitea, this file should not be shown as plain source only.