Document of the Java dependency of the emulators.

### [REQUIRED] Environment info

**firebase-tools:** 13.22.1

**Platform:** MacOS

### Issue:

This is mostly a documentation issue. If you try to run `firebase emulators:start` without Java installed, it will give you an error, but there's no mention of a java dependency in the docs or anywhere until you run it. And at that point, because I'm not a Java developer, I naively followed the instructions of the MacOS /usr/bin/java stub, and attempted to download Jave from [java.com](java.com) - which is not the correct place to get it, as I now know.

It would be great if some part of the docs mentioned the Java dependency and how to install a compatible version of Java (or even if the error message did - to which end, I did open a PR at https://github.com/firebase/firebase-tools/pull/7866 in case that's the best option, though documenting it in the installation instructions would also be helpful.)