chore: add 404 page

[platschi]

Subject of the issue

Add a 404 page to the repository.

Design

https://www.figma.com/file/mcaXc01qAA1si3I8shXkeI/Kwenta?type=design&node-id=3375-5335&mode=design&t=xH2Ke90TB045i0I5-4

Note

With IPFS, we might need to add a dedicated 404 error page

Adding 404 errors
If no file is found, IPFS will search for a file called ipfs-404.html in the requested path. This file can be used for showing 404 errors or redirecting. See the following blog post for more information: https://blog.ipfs.io/2020-06-26-go-ipfs-0-6-0/