Add a 404 Not Found Page

[TaiWilkin]

Overview

Adds a page for attempts to access routes which don't exist.

Connects #46

Demo

Testing Instructions

• Open the Netlify preview.
• Enter a route that doesn't exist, such as '/test'
• The 404 page should show
• Follow the link to home on the 404 page
• Traverse the real routes and confirm that the app works as before