feat(math): support local katex assets and improve docs (#742)

* refactor(math): update LaTeX guide and enhance KaTeX integration

- Revised the LaTeX documentation for clarity and improved structure, changing section titles and descriptions for better understanding.
- Added support for chemistry expressions using the mhchem extension.
- Introduced a new KaTeX CSS loader partial to streamline the integration of KaTeX, allowing for configurable remote or local asset loading.
- Updated the head partial to utilize the new KaTeX loader, enhancing the flexibility of math rendering options.

* refactor(mathjax): simplify MathJax script URL configuration

- Removed the dynamic version assignment for MathJax and set a fixed version in the script URL for consistency and clarity.

* docs(latex): enhance LaTeX guide and clarify chemistry expressions

- Updated section titles for better clarity, changing "Supported Functions" to "Chemistry Expressions."
- Improved description of the mhchem extension for rendering chemistry equations.
- Removed redundant instructions regarding the passthrough extension in Hugo.
- Corrected minor typographical errors in references to MathJax.

layouts/_partials/head.html

@@ -83,27 +83,11 @@
     }
   </script>
 
-  <!-- KaTeX-->
+  <!-- Math engine -->
   {{ $noop := .WordCount -}}
   {{- $engine := site.Params.math.engine | default "katex" -}}
   {{ if and (.Page.Store.Get "hasMath") (eq $engine "katex") -}}
-    <!-- TODO: make url configurable -->
-    {{ $katexBaseUrl := "https://cdn.jsdelivr.net/npm/katex@latest/dist" }}
-    {{ $katexCssUrl := printf "%s/katex%s.css" $katexBaseUrl (cond hugo.IsProduction ".min" "") -}}
-    {{ $katexFontPattern := "url(fonts/" }}
-    {{ $katexFontSubstituted := printf "url(%s/fonts/" $katexBaseUrl }}
-
-    {{ with try (resources.GetRemote $katexCssUrl) -}}
-      {{ with .Err -}}
-        {{ errorf "Could not retrieve KaTeX css file from %s. Reason: %s." $katexCssUrl . -}}
-      {{ else with .Value -}}
-        {{ $katexCssContent := strings.Replace .Content $katexFontPattern $katexFontSubstituted }}
-        {{ with resources.FromString (printf "css/katex%s.css" (cond hugo.IsProduction ".min" "")) $katexCssContent -}}
-          {{ $cssHash := . | fingerprint "sha512" -}}
-          <link rel="stylesheet" href="{{- .RelPermalink -}}" integrity="{{- $cssHash.Data.Integrity -}}" crossorigin="anonymous" />
-        {{ end -}}
-      {{ end -}}
-    {{ end -}}
+    {{ partialCached "scripts/katex.html" . -}}
   {{ else if and (.Page.Store.Get "hasMath") (eq $engine "mathjax") -}}
     {{ partialCached "scripts/mathjax.html" . -}}
   {{ end -}}