Fix documentation generation on Windows.

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>