dokka: Add links for external documentation

Provide the Javadoc URLs of dependencies which publish Javadoc to enable
dokka to create external links. The versions of the linked Javadoc might
not exactly match the versions of the dependencies, but this is still
better than broken links.

Signed-off-by: Martin Nonnenmacher <martin.nonnenmacher@here.com>

build.gradle

@@ -130,6 +130,24 @@ subprojects {
         input = files('src/main/kotlin', 'src/test/kotlin', 'src/funTest/kotlin')
     }
 
+    dokka {
+        externalDocumentationLink {
+            url = new URL('https://codehaus-plexus.github.io/plexus-containers/plexus-container-default/apidocs/')
+        }
+
+        externalDocumentationLink {
+            url = new URL('https://fasterxml.github.io/jackson-databind/javadoc/2.9/')
+        }
+
+        externalDocumentationLink {
+            url = new URL('http://jakewharton.github.io/DiskLruCache/')
+        }
+
+        externalDocumentationLink {
+            url = new URL('https://logback.qos.ch/apidocs/')
+        }
+    }
+
     task funTest(type: Test) {
         description = 'Runs the functional tests.'
         group = 'Verification'