Add 404 page

[WardBrian]

Submission Checklist

• Builds locally
• New functions marked with `r since("VERSION")`
• Declare copyright holder and open-source license: see below

Summary

Closes #451. Preview:

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):
Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[rok-cesnovar]

Looks good!