Add screenshots of the solution to the documentation

The documentation has no screenshots of the add-on in use, such as the Volto workflow control, the merged history, and the state badges. `sphinxcontrib-screenshot` is not wired into the docs build yet.

Start by locating the documentation build configuration and the pages describing the Volto workflow control, merged history, and state badges. Wire in sphinxcontrib-screenshot, add screenshots showing those features in use, and confirm the documentation build renders them successfully.

Written by the indexing model from the issue text.