Summary:
Several external sites link back to docs/navigator-comparison.html when talking about React Native's navigation. The Navigation guide added in #8390 is meant to replace this content, but it was added at docs/navigation.html.
This pull request removes the comparison guide and replaces it with the Navigation guide's content. There is no content update in this PR. For review purposes, note that the next link from the previous document (JS Environment) has been updated to point to navigator-comparison, and the content of the Navigation guide remain unchanged from #8390.
Closes https://github.com/facebook/react-native/pull/8417
Differential Revision: D3482273
Pulled By: caabernathy
fbshipit-source-id: 9e04e11a5829d48541f8612fb65c01fe319e768b