Properly handle docs pages without the .html extension

[robertlong]

Currently we don't rewrite the url to the correct file or even show a 404 page when the .html extension is omitted. We should fix this.

[LWprogramming]

I set the server up locally and a url like http://localhost:3000/docs/intro-avatars actually works fine, even though it doesn't work without the .html extension on the actual website. This setting seems related, but I haven't been able to figure out why it works locally even when cleanUrl is set to false, which is supposed to disable links without the .html extension.