The page at https://doc.nuxeo.com/nxdoc/document-templates-and-automation-rendering-service/ contains the link "MVEL manual" at the bottom which points to http://mvel.codehaus.org/Getting+Started+for+2.0 however the mvel.codehaus.org website does not exist any more.
I'm not sure how to replace the link. Perhaps the most relevant replacement site is:
however this appears to be mostly the Language Guide, and not Getting Started page, or the more friendly original home page, which is viewable at: https://web.archive.org/web/20081007000252/http://mvel.codehaus.org/
https://en.wikipedia.org/wiki/MVEL may also be useful.