Unfortunately there is no convention. If you want the documentation to be part of the generated site, obviously /src/site is a good place. Maybe you can even write the documentation in the APT format?
But more likely you have a set of doc, pdf and xls files, graphics, e-mails, etc. You can either place them under /src/site as well and put hyperlinks in the site or… define your own convention, sadly. Typically I have seen /src/main/doc(s), /src/doc(s) or even /doc(s).
You don’t want to place your documentation in /src/main/resources or src/main/webapp, as the files will then be part of built artifact (JAR/WAR) which is rarely desired.