Track velocity across several teams

The idea of the Cross-Team Velocity Chart is to aggregate velocity statistics across several teams that work together, presumably on the same product. These teams must align estimation approaches; their estimation of tasks of the same size is equal.

Other than the context described above, the behavior and interpretation of Cross-Team Velocity Chart statistics are the same as the article about tracking the velocity of a single team: https://brokenbuild.atlassian.net/wiki/spaces/VEL/pages/3124690945.