Add Theme Toggler to Docs Page

## **Feature Request: Add Theme Toggler to Docs Page**

## **Is your feature request related to a problem? Please describe.**
It can be difficult to read the documentation for long periods due to the lack of a dark/light mode option. This often causes eye strain, especially in low-light environments.

## **Describe the solution you'd like**
Add a simple **theme toggler** (light/dark mode switch) to the documentation page.
It could be placed in the top-right corner of the navbar and persist across pages and sessions.

## **Describe alternatives you've considered**
- Using browser extensions to invert colors (not ideal)
- Adjusting system or browser themes (inconsistent results)

## **Additional context**
A built-in theme toggler would greatly improve accessibility, comfort, and overall user experience when browsing the docs.

Jira issue: CRDB-56250