Previously doc/doc.css was a symlink to util/doc.css, but symlinks
don't work well on Windows. Now util/doc.css is copied to the output
directory when the documentation is generated.

Change-Id: I2c9f4fee4f4307cc3dd70c4be380b4551d5e9ab5
Reviewed-on: https://boringssl-review.googlesource.com/5677
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

Change-Id: I5175ba1b316e381949c820211769fbd636fe645e
Reviewed-on: https://boringssl-review.googlesource.com/3024
Reviewed-by: Adam Langley <agl@google.com>

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)