docs: rewrite dev containers documentation for GA
Corrects inaccuracies in SSH examples (deprecated `--container` flag),
port forwarding (native sub-agent forwarding is primary), and
prerequisites (dev containers are on by default). Fixes template
descriptions: docker-devcontainer uses native Dev Containers while
AWS/Kubernetes templates use Envbuilder.
Renames admin docs folder from `devcontainers/` to `envbuilder/` to
reflect actual content. Adds customization guide documenting agent
naming, display apps, custom apps, and variable interpolation. Documents
multi-repo workspace support and adds note about Terraform module
limitations with sub-agents. Fixes module registry URLs.
Refs #18907
Updates the dev containers documentation to accurately reflect that the
feature is generally available and document all configuration options.
Closes https://github.com/coder/internal/issues/1138
---
🤖 PR was written by Claude Sonnet 4.5 Thinking using [Coder
Mux](https://github.com/coder/cmux) and reviewed by a human 👩
This change documents the early access dev containers integration and
how to enable it, what features are available and what limitations exist
at the time of writing.
---------
Co-authored-by: EdwardAngert <17991901+EdwardAngert@users.noreply.github.com>
Mathias Fredriksson