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/css/chrome.css

@@ -560,7 +560,7 @@ ul#searchresults span.teaser em {
 
 .chapter li a.active {
   color: var(--sidebar-active);
-  background-color: rgba(8, 76, 207, 0.1);
+  background-color: var(--sidebar-active-bg);
 }
 
 .chapter li > a.toggle {