Generate a custom 404 page

[mbostock]

And maybe this could handle redirects for trailing slashes as described in #171.

[Fil]

Yes, we can handle #171 with the following script in 404.html, either by shooting in the dark when the requested URL ends with a /:

if (location.pathname.endsWith("/")) {
  location.replace(location.pathname.slice(0,-1) + ".html");
}

Or by listing only the relevant URLs:

if (["/javascript/"].find((dir) => location.pathname.endsWith(dir)) {
  location.replace(location.pathname.slice(0,-1) + ".html");
}

We could build 404.html from a custom 404.md, if present.

[Fil]

A --base (#42) will be useful for the built 404.html, as it might be served from different depths in /the/request/path/hierarchy.