The javadoc documentation contains a couple of good examples. It always points to the root of the documentation, so if there is something you want to include on each page, you use it to find the javadoc root, since the relative path will differ from page to page
For example, to include your company logo at the top of each created documentation page, you can put it logo.pngin the root folder of the documentation, and then add:
<img src="{@docRoot}/logo.png">
in javadoc header