Try to serve index.html if link points to directory

[almielczarek]

This change may fix a lot of broken links such as the ones I've found in hyperium/hyper#1749 and hyperium/hyper#1750

[almielczarek]

I peeked at web::rustdoc::rustdoc_html_server_handler and it looks like it checks if a file exists at the request path and if it doesn't it immediately returns a 404.

Perhaps we could try pushing "index.html" to the end of the path once in the None branch of this match block and check if that new path exists before returning a 404.

[jyn514]

We currently add index.html for /, but not if the slash is missing. Thanks for writing up instructions!

[d-e-s-o]

Just hit the same issue with tokio docs. Wanted to see documentation for the https://docs.rs/tokio/0.2.11/tokio/time but "the resource does not exist". Adding slash or /index.html works, but if you don't know that you are basically forced to try to navigate from the project's documentation root to find what you need, which is rather tedious.