New 404

[kimsauce]

Purpose of this pull request

New 404 page. This is a backup solution and will be implemented only if default Docusaurus 404 revert fails to send events to GA.

Select the type of change:

• [ ] Minor Changes - Typos, formatting, slight revisions, .clabot
• [ ] Update Content - Revisions, updating sections
• [ ] New Content - New features, sections, pages, tutorials
• [x] Site and Tools - Updates, maintenance, dependencies, new packages for the site (Docusaurus, Gatsby, React, etc.)