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/general.css

@@ -113,7 +113,7 @@ h6:target::before {
 */
 :target {
   /* Safari does not support logical properties */
-  scroll-margin-top: calc(var(--menu-bar-height) + 0.5em);
+  scroll-margin-top: calc(var(--menu-bar-height) + 2rem);
 }
 
 .page {
@@ -141,7 +141,7 @@ h6:target::before {
 
 .content {
   overflow-y: auto;
-  padding: 24px 4px 48px 4px;
+  padding: 48px 4px;
 }
 .content main {
   margin-inline-start: auto;