Add context page for API docs

[harinikmsft]

trafficstars

It would be good to add a page that sets context on what the API docs section is doing - point to RN docs for all RN APIs, set context on Windows specific work, Mac specific work etc., Right now it is confusing to land in that section without context.

[chrisglein]

Agreed. It pretty unceremoniously drops you into a specific API: https://microsoft.github.io/react-native-windows/docs/flyout-component Which is what the Facebook docs do, but theirs are more comprehensive.

[harinikmsft]

There is a good context page for the react native API docs now.

[crutchcorn]

I'd be happy to write this if that would be helpful