Located at http://userguide.icu-project.org
We might copy the entire userguide to a new location for each release, so we save a 'snapshot'.
To mirror the entire site using wget, you can use the following command. The results won't be very pretty (stylesheets are missing, etc), but it will be self contained.
A better method seems to be using the Google Sites API, for example via the Java tool from the Google Data Liberation team: