Destination URLs always end with `/index.html` when building with `dirhtml`

[jahn-junior]

When using the dirhtml builder, destination URLs always end with /index.html. As far as I know, this is redundant and can be omitted to clean up the links.

I propose adding an option to omit this from the destination URLs. I think it's too opinionated to be the default behavior, but it'd be nice if my team could opt-in to the simpler URLs.