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/book.toml

@@ -9,6 +9,8 @@ site-url = "/docs/"
 [output.html]
 no-section-label = true
 preferred-dark-theme = "light"
+additional-css = ["theme/page-toc.css"]
+additional-js  = ["theme/page-toc.js"]
 
 [output.html.print]
 enable = false