How do i install Javadoc?

Locate the build.gradle file in your project and add the following lines:

repositories {
    mavenCentral()
}

def jme3 = [grp: 'org.jmonkeyengine', ver: '3.4.0-stable']

dependencies {
    // jMonkeyEngine dependencies
    implementation "${jme3.grp}:jme3-core:${jme3.ver}"
    implementation "${jme3.grp}:jme3-desktop:${jme3.ver}"
    implementation "${jme3.grp}:jme3-effects:${jme3.ver}"
    implementation "${jme3.grp}:jme3-jbullet:${jme3.ver}"
    implementation "${jme3.grp}:jme3-jogg:${jme3.ver}"
    implementation "${jme3.grp}:jme3-lwjgl3:${jme3.ver}"
    implementation "${jme3.grp}:jme3-niftygui:${jme3.ver}"
    implementation "${jme3.grp}:jme3-plugins:${jme3.ver}"
    implementation "${jme3.grp}:jme3-terrain:${jme3.ver}"
    implementation "${jme3.grp}:jme3-testdata:${jme3.ver}"
}

Note that you might not need all of these dependencies in your project.