Create 404.html for doc site

[Tacktician]

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Description

Created a default 404.html that pauses, and redirects to the home page of doc site upon receiving a 404 response code

Motivation and Context

Temporary fix for folks who bookmarked the old overview.html index page

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [ ] I have read the contributing document.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[Tacktician]

@KunalJain, I wasn't able to test this locally because we're using docusaurus v1 and GitHub pages for the website. V2 has a better workaround for these issues.