Reduce spacing between nav items (#5310)

**Before**
<img width="1475" alt="Screenshot 2024-07-22 at 3 56 46 PM"
src="https://github.com/user-attachments/assets/a9c6b929-0569-45a6-9840-d49d955dd55a">

--------

**After**
<img width="1475" alt="Screenshot 2024-07-22 at 3 56 38 PM"
src="https://github.com/user-attachments/assets/98c85e03-fe62-4894-99f6-a49293eacf0b">

docs/stylesheets/extra.css

@@ -103,3 +103,12 @@
   color: var(--md-code-fg-color);
   background-color: var(--md-code-bg-color);
 }
+
+/* Reducing spacing between nav items to fit more content */
+.md-nav__link {
+  margin-top: 0.3em;
+}
+/* Retain larger spacing for the right-side table-of-contents nav */
+.md-nav--secondary .md-nav__link {
+  margin-top: 0.5em;
+}