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

Start with the current backend devcontainers plugin processor and its documentation, then compare their export patterns and setup guidance with Backstage's New Backend System documentation. Done means the exports follow the applicable pattern and the documentation covers both the classic and new backend systems.

Written by the indexing model from the issue text.