Connected to #63

Changes made

• Updates the backstage-plugin-devcontainers-backend READMEs to remove all APIs that no longer exist
• Adds disclaimers to each plugin's directory README to indicate what version the documentation is based on

Notes

• I do think that we'll need to figure out versioned docs once we're stable enough to hit v1.0.0, but because we're so early, it felt better to add a disclaimer that things are subject to change a lot as we figure things out
  • I've also been much more deliberate about highlighting breaking changes for each new release