website: render the design essays' TeX (math: true, mathjax3 pinned to v4)

design/revertible-effects and design/context-model carry real TeX that was
showing as literal $$ source. markdown: { math: true } enables
markdown-it-mathjax3; pinned ^4.3.2 deliberately — v5 injects a <style> tag
per formula (31 on one page), which Vue's template compiler rejects, so v5
cannot work under VitePress; v4 emits pure SVG (probed: 0 style tags).

website/package.json

@@ -9,6 +9,7 @@
     "preview": "vitepress preview ."
   },
   "devDependencies": {
+    "markdown-it-mathjax3": "^4.3.2",
     "vitepress": "^1.6.3",
     "vue": "^3.5.13"
   }