You should create a FAQ entry, describing what license requirements are enforced on a website by including piwik.js and how the GPLv3 should be interpreted.
Using piwik.js is different from redistributing piwik.js, so there's no problem.
For example, you can [b]use[/b] Apache with PHP on a virtualized Linux environment on a Windows server, with BSD libraries.
You would not be able to redistribute the above under a single license or as a public domain collection.
A link exists to the GPL on the FSF site. We shouldn't try to simplify the legalese via FAQ.