Fix line splitting in `white-space: pre` flows

The newline position from one line was getting used repeatedly, causing extra
"phantom" newlines in the following lines.  Fixes #3413.
Fix http://doc.servo.org/ to redirect to /servo/index.html again...

... rather than being the Rust documentation index.

The order of `cp` command determines which `target/doc/index.html` file
overwrites the other.
Merge pull request #3414 from mbrubeck/pre-line-break

Fix line splitting in `white-space: pre` flows. r=gw