Add a new page documenting React Native's out-of-tree platform support

[empyrical]

This PR adds a new page to the React Native docs that showcases other platforms that have been developed for React Native, and a bit documenting the API change that was added in facebook/react-native#20825 . In the future, this should be able to be expanded with more information on making/maintaining React Native platforms.

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[matthargett]

I would mention haul in addition to metro.
It would be great to include the first couple of error messages (e.g. when Accessibility.platform.js is missing) and how to fix them.

[empyrical]

I almost wonder if I should make a PR that throws an error if you pass a --platform that doesn't exist. Also Haul would be a good thing to mention, yes. But IIRC it only supports iOS/Android - a PR to remedy that could probably be made too!

[kelset]

once the typo is fixed I think this can be merged. I'll reply to what Matt wrote separately

[kelset]

related to

It would be great to include the first couple of error messages (e.g. when Accessibility.platform.js is missing) and how to fix them.

We have been discussing about adding a "known issue" section to each component/page - you can check out how it would look here -> #519

So I think that if you want to add it you should follow that "approach"