Add custom 404

[lachellel]

Description of Issue:

We are proposing changing a URI (/applications to /components)

Details of Issue:

If we make this change and there are embedded links in other docs, we should include an updated/custom 404 as well

References (Docs, Links, Files):

It doesn't seem like we added a custom 404 yet to this repo.
Simple example: https://github.com/GSA/fpki-guides/blob/staging/404.md
Federalist guide for custom 404's: https://federalist.18f.gov/documentation/customization/

[idmken]

Included in new playbooks site.