Fix blank top-level 404 page

[thejcannon]

Fixes #7 in an unfortunately clever, yet still appropriate way.

See the comment in src/js/docsPluginWithTopLevel404.js for greater detail.

Tests:

• http://localhost:3000 -> ✔️
• http://localhost:3000/ -> (redirects to http://localhost:3000)
• http://localhost:3000/2.20/docs/introduction/welcome-to-pants -> ✔️
• http://localhost:3000/2.20/nope -> (Page Not Found) ✔️
• http://localhost:3000/nope -> (Page Not Found) ✔️
• http://localhost:3000/sponsorship -> ✔️

[huonw]

Interest! It's not the worst; happy to try it for now.

I tested some extra URLs, e.g. embedded within the docs http://localhost:3000/2.20/reference/wrong ✅