Summary:
Restored the version lag caveat in the Getting Started, and updated the Upgrading guide to account for CRNA.
Additionally, updated the release versions page to account for the new release cycle (e.g. release candidates are encouraged).
Closes https://github.com/facebook/react-native/pull/14112
Differential Revision: D5112224
Pulled By: hramos
fbshipit-source-id: d544c5178484c27f1ed6dee3479bcd94e5e81982