feat(#2112): enhance 404 page with branding, search, and helpful links

[imtushar01]

Problem

The current 404 page only displays “404 - This page could not be found” without navigation, search, or branding. As a result, users cannot:

• Identify that they are on a CHT documentation page
• Navigate back to relevant content
• Search for the information they need

Solution

Added a custom layouts/404.html page aligned with the existing documentation layout:

• Integrated sidebar navigation and top search bar
• Included CHT branding for consistency
• Added helpful guidance and links for users

Fixes #2112

[mrjones-plip]

Thanks so much @imtushar01 ! I simplified the language a bit, but this is good to go.