docs: add dev container screenshots (#21191)

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- Outdated status with rebuild option (working-with-dev-containers.md)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Also deletes the outdated devcontainer-agent-ports.png.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

docs/user-guides/devcontainers/customizing-dev-containers.md

@@ -84,6 +84,8 @@ appears in `coder ssh` commands and the dashboard (e.g.,
 Control which built-in Coder apps appear for your dev container using
 `displayApps`:
 
+![Dev container with all display apps disabled](../../images/user-guides/devcontainers/devcontainer-apps-bar.png)_Disable built-in apps to reduce clutter or guide developers toward preferred tools_
+
 ```json
 {
   "name": "My Dev Container",

docs/user-guides/devcontainers/index.md

@@ -11,6 +11,8 @@ administrators can configure
 [Envbuilder](../../admin/integrations/devcontainers/envbuilder/index.md) instead,
 which builds the workspace image itself from your dev container configuration.
 
+![Two dev containers running as sub-agents in a Coder workspace](../../images/user-guides/devcontainers/devcontainer-running.png)_Dev containers appear as sub-agents with their own apps, SSH access, and port forwarding_
+
 ## Prerequisites
 
 - Coder version 2.24.0 or later
@@ -67,6 +69,8 @@ Coder automatically discovers dev container configurations in your repositories
 and displays them in your workspace dashboard. From there, you can start a dev
 container with a single click.
 
+![Discovered dev containers with Start buttons](../../images/user-guides/devcontainers/devcontainer-discovery.png)_Coder detects dev container configurations and displays them with a Start button_
+
 If your template administrator has configured automatic startup (via the
 `coder_devcontainer` Terraform resource or autostart settings), your dev
 container will build and start automatically when the workspace starts.

docs/user-guides/devcontainers/working-with-dev-containers.md

@@ -3,7 +3,7 @@
 The dev container integration appears in your Coder dashboard, providing a
 visual representation of the running environment:
 
-![Dev container integration in Coder dashboard](../../images/user-guides/devcontainers/devcontainer-agent-ports.png)
+![Two dev containers running as sub-agents in a Coder workspace](../../images/user-guides/devcontainers/devcontainer-running.png)_Dev containers appear as sub-agents with their own apps, SSH access, and port forwarding_
 
 ## SSH access
 
@@ -152,4 +152,6 @@ When you modify your `devcontainer.json`, you need to rebuild the container for
 changes to take effect. Coder detects changes and shows an **Outdated** status
 next to the dev container.
 
+![Dev container showing Outdated status with rebuild option](../../images/user-guides/devcontainers/devcontainer-outdated.png)_The Outdated indicator appears when changes to devcontainer.json are detected_
+
 Click **Rebuild** to recreate your dev container with the updated configuration.