rustdoc should output rust.css

[brson]

The HTML rustdoc outputs expects to find rust.css for styling but does not provide it. #1654 would help

[bstrie]

Fingers crossed, this would probably be obviated by @cmr's rustdoc revamp.

[emberian]

#8125

rustdoc_ng makes this entirely irrelevant.

[emberian]

Now that rustdoc_ng has been merged, closing.