docs: clarify dev containers entry point and reduce callouts (#21188)

The user guide jumped straight into integration details without explaining
what dev containers are. Now it opens with a brief orientation linking to
the spec, then explains this guide covers the Docker-based approach.

Converted several NOTE callouts to prose where they were just cross-references
or stacked unnecessarily. The Envbuilder index note was reframed to lead with
its strengths rather than "we recommend the other thing."

Also updates platform support to Linux only per current status.

Refs #21157

docs/admin/integrations/devcontainers/envbuilder/index.md

@@ -6,13 +6,12 @@ Unlike the [Dev Containers integration](../integration.md),
 Envbuilder transforms the workspace image itself rather than running containers
 inside the workspace.
 
-> [!NOTE]
->
-> For most use cases, we recommend the
-> [Dev Containers integration](../integration.md),
-> which uses the standard `@devcontainers/cli` and Docker. Envbuilder is an
-> alternative for environments where Docker is not available or for
-> administrator-controlled dev container workflows.
+Envbuilder is well-suited for Kubernetes-native deployments without privileged
+containers, environments where Docker is unavailable or restricted, and
+workflows where administrators need infrastructure-level control over image
+builds, caching, and security scanning. For workspaces with Docker available,
+the [Dev Containers Integration](../integration.md) offers container management
+with dashboard visibility and multi-container support.
 
 Dev containers provide developers with increased autonomy and control over their
 Coder cloud development environments.