CBL-Java : Document installation changes

Beginning with the next CBL release (3.1.12?), the documented way of working with CBL Java will change (currently documented here: https://docs.couchbase.com/couchbase-lite/current/java/gs-prereqs.html ).

1. On all platforms, the Downloads page will point to https://docs.couchbase.com/couchbase-lite/current/java/gs-install.html . This page, and the prereqs page https://docs.couchbase.com/couchbase-lite/current/java/gs-prereqs.html ,  should instruct to set up their Maven / Gradle projects referencing these repositories, rather than utilizing the .jar files from the downloaded .zip.
2. On Linux, there will be a new downloadable artifact named couchbase-lite-java-linux-supportlibs-<X.Y.Z>.zip . The full URL for that package will be eg. https://packages.couchbase.com/releases/couchbase-lite-java/3.1.3/couchbase-lite-java-linux-supportlibs-3.1.3.zip . The documentation should link to this download directly, and users should be told to use the .so files in that .zip in a fashion similar to what is documented today.