Issue 5805 - Documentation images

Now the plantuml generated images will be in the `images/` directory
and any external image linked should also be saved in the `images/` directory.

Unfortunately this does not render well in the IntelliJ editor,
but it renders correctly in the HTML documentation.

Signed-off-by: Simone Bordet <simone.bordet@gmail.com>

documentation/jetty-documentation/src/main/asciidoc/operations-guide/.asciidoctorconfig

@@ -1,4 +1,5 @@
 // Asciidoctor IDE configuration file.
 // See https://github.com/asciidoctor/asciidoctor-intellij-plugin/wiki/Support-project-specific-configurations
 :experimental:
+:imagesdir: images
 :JETTY_HOME: ../../../../../../../jetty-home/target/jetty-home