I think it will be good to be able to switch between Piwik versions. At least between Piwik 2 and 3.

For auto generated docs it's actually not too hard I think to generate documentation for different versions as we used to do this already but never exposed it to the UI. There were maybe a view problems though.

We would also need this for the regular guides though. So the task would be to move all docs into a subfolder that contains the Piwik version. Eg docs/2.0/... and docs/3.0/... ideally it would at some point work for all patch versions and whenever there is a new release we simply older docs unchanged and make changes only on new version. Lots of duplicated content though so we need to make sure maybe to only index the docs for latest version.

As of the UI we will need to add a version selector and likely even have more "speaking" URLs. Ideally it was something like /3.0/guides/... where /guides/... would point to latest version to keep links active. However, for simplicity, we might instead save the selected Piwik in a cookie instead. Not sure if this is good or bad for SEO, maybe rather bad as it might not index correct version or something? I think as long as we serve the latest version by default it might be ok. For now I'll add a version selector next to "Search"