Measure and document core feature parity

[chrisglein]

Proposal: Document which React Native APIs are and are not supported by React Native for Windows

Details

We've previously tracking core parity with various one-off issues.
#2136
#2113
#2111
#2796
#3016
These were built from canvasing partners and getting a rough breakdown of priority based on immediate need.

As tracking the long list of issues with sub-issue checkboxes became unwieldy I split those out into issues and created project boards to track overall progress and ordering:
https://github.com/microsoft/react-native-windows/projects/17
https://github.com/microsoft/react-native-windows/projects/18
https://github.com/microsoft/react-native-windows/projects/19
https://github.com/microsoft/react-native-windows/projects/20
https://github.com/microsoft/react-native-windows/projects/21
https://github.com/microsoft/react-native-windows/projects/22

There's a couple parts here:

1. Make sure we don't miss any APIs
2. Document our implemented/not-implemented state on all APIs

We want an easy to consult source of truth on #2 that has a reasonable plan for how to keep up to date (at least until we have a plan to integrate that status directly into the official RN docs). What form it takes is less important. It could be a spreadsheet, a doc on the wiki, a file checked into the repo. Savoy created a detailed approach for Accessibility here which is a good reference: #3016

Simplifying that to remove tracking of iOS/Android, Current vs. vNext, RN59 vs. RN90... we could be fine with just this:

API	Status
Text Props
accessible	Yes
accessibilityHint	Yes
accessibilityLabel	Yes
accessibilityRole	#3111
accessibilityState	#3111

Where each API has these possible states:

• Implemented
• Not implemented, with link to issue tracking future implementation
• Never will be implemented, with link to discussion (or that information is provided inline)

FYI, many (but not all) of the issues are being tracked with this label: https://github.com/microsoft/react-native-windows/labels/API%20Completion
That may help seed, but we really need a fresh diff from the official RN docs as of RN60

Note that we want fine-grained detail here, but whatever solution we land here should also roll up into this: https://github.com/microsoft/react-native-windows/blob/master/vnext/docs/ParityStatus.md