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
draft: add contribution docs for modules and templates individually to
be referenced in coder docs manifest.
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Co-authored-by: Atif Ali <atif@coder.com>
Mathias Fredriksson