Update eXist-db installation instructions

[djbpitt]

We can specify that users who don't have Java can get easy-to-install binaries at https://adoptopenjdk.net/. Aside from all of the other reasons to prefer open source, OpenJDK doesn’t require registration, which Oracle does.

Windows users who install the win-zip distribution of eXist-db may not know whether they have Java or not, so our instructions should include information about how to check.