Add figure with overview of Carpentries community governance

### How could the content be improved?

In a community governance discussion call today, @brownsarahm suggested that the attached figure could be helpful to include in the Instructor Training curriculum. I guess it would belong in _How We Operate_? Happy to discuss and update the figure as needed, if you agree that it should find a place in the curriculum.

### Which part of the content does your suggestion apply to?

https://carpentries.github.io/instructor-training/15-carpentries.html

Start by opening the linked Instructor Training page, especially the How We Operate section, and inspect how figures are included in the curriculum. Confirm the figure’s placement and any needed updates with maintainers, then render or preview the lesson to verify it appears correctly.

Written by the indexing model from the issue text.