docs: restructure dev container documentation (#21157)

Dev container admin docs were scattered across two locations: the Docker-based
integration under extending-templates/ and Envbuilder under managing-templates/.
There was no landing page explaining that two approaches exist or helping admins
choose between them.

This moves everything under admin/integrations/devcontainers/ with a decision
guide at the top. Dev containers are an integration with the dev container
specification, so integrations/ is a natural fit alongside JFrog, Vault, etc.

Stub pages remain at the original locations for discoverability.

New structure:

  admin/integrations/devcontainers/
  ├── index.md                                # Landing page + decision guide
  ├── integration.md                          # Docker-based dev containers
  └── envbuilder/
      ├── index.md
      ├── add-envbuilder.md
      ├── envbuilder-security-caching.md
      └── envbuilder-releases-known-issues.md

Refs #21080

docs/admin/integrations/devcontainers/index.md

@@ -0,0 +1,49 @@
+# Dev Containers
+
+Dev containers allow developers to define their development environment
+as code using the [Dev Container specification](https://containers.dev/).
+Configuration lives in a `devcontainer.json` file alongside source code,
+enabling consistent, reproducible environments.
+
+By adopting dev containers, organizations can:
+
+- **Standardize environments**: Eliminate "works on my machine" issues while
+  still allowing developers to customize their tools within approved boundaries.
+- **Scale efficiently**: Let developers maintain their own environment
+  definitions, reducing the burden on platform teams.
+- **Improve security**: Use hardened base images and controlled package
+  registries to enforce security policies while enabling developer self-service.
+
+Coder supports two approaches for running dev containers. Choose based on your
+infrastructure and workflow requirements.
+
+## Dev Containers Integration
+
+The Dev Containers Integration uses the standard `@devcontainers/cli` and Docker
+to run containers inside your workspace. This is the recommended approach for
+most use cases.
+
+**Best for:**
+
+- Workspaces with Docker available (Docker-in-Docker or mounted socket)
+- Dev container management in the Coder dashboard (discovery, status, rebuild)
+- Multiple dev containers per workspace
+
+[Configure Dev Containers Integration](./integration.md)
+
+For user documentation, see the
+[Dev Containers user guide](../../../user-guides/devcontainers/index.md).
+
+## Envbuilder
+
+Envbuilder transforms the workspace image itself from a `devcontainer.json`,
+rather than running containers inside the workspace. It does not require
+a Docker daemon.
+
+**Best for:**
+
+- Environments where Docker is unavailable or restricted
+- Infrastructure-level control over image builds, caching, and security scanning
+- Kubernetes-native deployments without privileged containers
+
+[Configure Envbuilder](./envbuilder/index.md)