Add KaTeX for rendering maths in pkgdown site. NOTE: this should be e…

…asier but see r-lib/pkgdown#2704 and update this when fixed

_pkgdown.yml

@@ -1,6 +1,11 @@
 url: https://mlmc.louisaslett.com/
 template:
   bootstrap: 5
+  includes:
+    in_header: |
+      <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/[email protected]/dist/katex.min.css" integrity="sha384-nB0miv6/jRmo5UMMR1wu3Gz6NLsoTkbqJghGIsx//Rlm+ZU03BU6SQNC66uf4l5+" crossorigin="anonymous">
+      <script defer src="https://cdn.jsdelivr.net/npm/[email protected]/dist/katex.min.js" integrity="sha384-7zkQWkzuo3B5mTepMUcHkMB5jZaolc2xDwL6VFqjFALcbeS9Ggm/Yr2r3Dy4lfFg" crossorigin="anonymous"></script>
+      <script defer src="https://cdn.jsdelivr.net/npm/[email protected]/dist/contrib/auto-render.min.js" integrity="sha384-43gviWU0YVjaDtb/GhzOouOXtZMP/7XUzwPTstBeZFe/+rCMvRwr4yROQP43s0Xk" crossorigin="anonymous" onload="renderMathInElement(document.body);"></script>
 navbar:
   structure:
     left: [intro, reference, articles, tutorials, news, giles]