Rename the file camel-quarkus-integration-test-vertx-websocket-3.0.0-RC2-javadoc.jar to camel-quarkus-integration-test-vertx-websocket-3.0.0-RC2-javadoc.zip
Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder camel-quarkus-integration-test-vertx-websocket-3.0.0-RC2-javadoc
Double click index.html will open the index page on your default web browser.