Devcontainers backend plugin: update code/documentation to account for Backstage's 'New Backend System'

The current code/documentation for the backend devcontainers plugin is based on Backstage's previous backend system. But recently, they've rolled out a [New Backend System](https://backstage.io/docs/plugins/new-backend-system/)

I don't think we'll actually need to make any changes to the processor class itself, but we do want to make sure that:
1. The class follows any new patterns for exports
2. The documentation is updated to walk somebody through adding the devcontainers plugin for the old/classic backend system, and the new one