Prepare documentation for 1.0 release

[ethan-kusters]

Bug/issue #, if applicable: rdar://90124447

Summary

Updates documentation throughout to prepare for the 1.0 release of the plugin aligned with Swift 5.6.

Testing

Confirm that documentation doesn't contain any references to the main branch or the plugin being under active-development and not ready for production use.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [ ] Added tests Non-functional change.
• [x] Ran the ./bin/test script and it succeeded
• [x] Updated documentation if necessary

[ethan-kusters]

@swift-ci please test