[To Do]: Update 404 not found error page

[peteGSX]

Task description

Critical path for documentation refresh:

We must revisit the new 404.html to tell it where to find the old site. Can we do that with the same code that changes all the old links we have just added to the new one?

I think we may need to edit the EX-CSB1 and EX8874 QR code redirects as well.