Add KaTeX to rendered docs. (#832)

This adds a single test formula to the `zebra-chain` docs.  We can remove it
after merging this and running it through CI to confirm that it works.

.github/workflows/firebase.yml

@@ -43,6 +43,8 @@ jobs:
     - name: Build external docs
       run: |
         cargo doc --no-deps
+      env:
+        RUSTDOCFLAGS: "--html-in-header katex-header.html"
 
     - name: Deploy external docs to firebase
       uses: w9jds/firebase-action@v1.5.0
@@ -55,6 +57,8 @@ jobs:
     - name: Build internal docs
       run: |
         cargo doc --no-deps --document-private-items
+      env:
+        RUSTDOCFLAGS: "--html-in-header katex-header.html"
 
     - name: Deploy external docs to firebase
       uses: w9jds/firebase-action@v1.5.0

katex-header.html

@@ -0,0 +1,8 @@
+<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/katex@0.12.0/dist/katex.min.css" integrity="sha384-AfEj0r4/OFrOo5t7NnNe46zW/tFgW6x/bCJG8FqQCEo3+Aro6EYUG4+cU+KJWu/X" crossorigin="anonymous">
+<script defer src="https://cdn.jsdelivr.net/npm/katex@0.12.0/dist/katex.min.js" integrity="sha384-g7c+Jr9ZivxKLnZTDUhnkOnsh30B4H0rpLUpJ4jAIKs4fnJI+sEnkvrMWph2EDg4" crossorigin="anonymous"></script>
+<script defer src="https://cdn.jsdelivr.net/npm/katex@0.12.0/dist/contrib/auto-render.min.js" integrity="sha384-mll67QQFJfxn0IYznZYonOWZ644AWYC+Pt2cHqMaRhXVrursRwvLnLaebdGIlYNa" crossorigin="anonymous" onload="renderMathInElement(document.body);"></script>
+<style>
+.katex { font-size: 1em !important; }
+pre.rust, .docblock code, .docblock-short code { font-size: 0.85em !important; }
+</style>
+

zebra-chain/src/lib.rs

@@ -1,4 +1,6 @@
 //! Blockchain-related datastructures for Zebra. 🦓
+//!
+//! $$a^2 + b^2 = c^2$$
 
 #![doc(html_favicon_url = "https://www.zfnd.org/images/zebra-favicon-128.png")]
 #![doc(html_logo_url = "https://www.zfnd.org/images/zebra-icon.png")]