This is Kinovea's user-documentation project.
Kinovea is an open source video player and camera capture software with features tailored to sports applications such as comparison, annotation and measurement. It is a relatively mature project, it has been in development since 2004, has been downloaded more than a million times and is cited in many scientific papers in the field of sports science.
Main website: https://www.kinovea.org/
The current live documentation can be found here: https://www.kinovea.org/help/en/index.htm
List of successive release posts since version 0.8.15, containing descriptions of new features (some of the features have seen multiple updates over the course of these versions):