docs: update Swift gazelle plugin reference to correct repository (#2159)

## Summary
- Update Swift gazelle plugin reference name from
`rules_swift_package_manager` to `swift_gazelle_plugin`
- Update repository URL to correct location:
`https://github.com/cgrindel/swift_gazelle_plugin`

## Test plan
- [x] Verify all references to the old repository name have been updated
- [x] Confirm the new URL is accessible and correct

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Co-authored-by: Claude <noreply@anthropic.com>