doc(readme): remove iOS docs (migrated to cordova-docs)

[erisu]

Motivation and Context

Splash Screen has been migrated as a core feature of the iOS platform. This PR is to remove the documentation from the plugin repo and remove any confusion that the plugin still supports iOS.

The iOS related documentation is being migrated to cordova-docs

Description

Cleanup README

Testing

Checklist

• [ ] I've run the tests to see all new and existing tests pass
• [ ] I added automated test coverage as appropriate for this change
• [ ] Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• [ ] If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• [ ] I've updated the documentation if necessary