Extend https://docs.sentry.io/platforms/react-native/ with an Expo subpage

[alwx]

Maybe it could be done the same way as it's done in the case of Flutter where Flutter is a subpage of Dart in the docs. The goal here is to bring more visibility to Expo and indicate it one more time that we support Expo and have integrations with different Expo components.

[linear-code]

RN-670