Add screenshots to the dev containers user guide:
- Running dev containers with sub-agents (index.md, working-with-dev-containers.md)
- Discovered dev containers with Start button (index.md)
- Outdated status with rebuild option (working-with-dev-containers.md)
- Display apps disabled (customizing-dev-containers.md)
Also deletes the outdated devcontainer-agent-ports.png.
Refs #21157
Mathias Fredriksson
