Summary:We don't support the latest NDK (`r11b`) and Google no longer provides links for the old version on the official page. So add them to the docs for now.
cc kmagiera astreet
Closes https://github.com/facebook/react-native/pull/6668
Differential Revision: D3102125
fb-gh-sync-id: 0a54e1c7f2ff883ca516c30cda279c828bd5a6e2
fbshipit-source-id: 0a54e1c7f2ff883ca516c30cda279c828bd5a6e2