Update readmes

[franziskuskiefer]

Update docs in the repo to ensure they clearly communicate content and status

• add readmes to all crates
• add verification status
• update top level readme to reflect the current state
  • add a legend of sorts to the top-level README

[github-actions]

This issue has been marked as stale due to a lack of activity for 60 days. If you believe this issue is still relevant, please provide an update or comment to keep it open. Otherwise, it will be closed in 7 days.

[franziskuskiefer]

@jschneider-bensch I think we can close this? There's #569 as a follow up.