docs: Update README and CI in preparation for semantic 1.0.0 release

[scottmarchant]

Summary

A few small updates to the readme and CI, ahead of making a 1.0.0 release:

• Removed language about the experimental nature of this project. Enough testing has been done to feel confident in the public-facing API and functionality of swift-dispatch-async
• Re-ordered some sections in readme to elevate usage instructions
• Cleared out a stale todo item
• Pinned referenced GitHub swift workflows to 0.0.2, as 0.0.3 changes are unstable.