test: add smoke test

Author: mcanouil

tests/docs/smoke-all/2025/05/18/issue12766.qmd

@@ -0,0 +1,26 @@
+---
+format:
+  html-default:
+    output-file: default
+    html-math-method: mathjax
+  html-math:
+    output-file: math
+    html-math-method:
+      method: mathjax
+_quarto:
+  tests:
+    html-default:
+      ensureFileRegexMatches:
+        - ['\\tag\{1\}\\]'] 
+        - []
+    html-math:
+      ensureFileRegexMatches:
+        - ['\\tag\{1\}\\]'] 
+        - []
+---
+
+$$
+\alpha = \beta
+$$ {#eq-ab}
+
+This is @eq-ab.