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
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
Danielle Maywood