Templates: Update autogenerated docs to include `platforms` detail

When templates were located in [microsoft/vscode-dev-containers](https://github.com/microsoft/vscode-dev-containers), the `platforms` variable was used for the auto-generated README files.

[Example](https://github.com/microsoft/vscode-dev-containers/tree/main/containers/azure-functions-dotnetcore-3.1) 👇


Looks like we did migrate the variable so that it could be utilized for auto-generated docs, however, it seems to have slipped though the gaps - I'll update [devcontainers/action](https://github.com/devcontainers/action) to utilize it. Wondering, @jcbhmr would that be helpful?

_Originally posted by @samruddhikhandale in https://github.com/devcontainers/templates/issues/145#issuecomment-1515538661_