docs: Add table of contents navigation (#15212)

To ease navigating on pages that are long and having a birds-eye view of
all the available content.

This is done client-side and done via the files initially generated by
the `mdbook-pagetoc` plugin ([crates.io link
here](https://crates.io/crates/mdbook-pagetoc)).

<img width="600" alt="Screenshot 2024-07-25 at 13 34 08"
src="https://github.com/user-attachments/assets/a78c69e5-8cc4-4414-9d9c-27a4ceb27620">

---

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

docs/theme/index.hbs

@@ -119,7 +119,6 @@
         </script>
 
         <div id="page-wrapper" class="page-wrapper">
-
             <div class="page">
                 {{> header}}
                 <div id="menu-bar-hover-placeholder"></div>
@@ -192,7 +191,12 @@
 
                 <div id="content" class="content">
                     <main>
-                        {{{ content }}}
+                      <div class="sidetoc">
+                          <nav class="pagetoc">
+                            <p class="toc-title">On this page</p>
+                          </nav>
+                      </div>
+                      {{{ content }}}
                     </main>
                 </div>
             </div>