I find that the Javadoc link from the hub( ie, the link at the top of this page ) is not opening. As I remember it hasen’t been opening since atleast 1 or two days. If I click on it a generic git hub error page “There isn’t a GitHub Pages site here” opens up.
You can access to JavaDoc from SDK.
first download and install SDK from Releases · jMonkeyEngine/sdk · GitHub
then set your default browser from Tools–> Options–>General–>WebBrowser to see docs in browser. now go to: