[docs] "Existing React Native apps" and "Existing native apps" docs groups

[brentvatne]

• Rename "Bare React Native" to "Existing React Native apps"
• Stopped telling people how to initialize bare apps there.
• We should do more work to integrate instructions for how to configure libraries for bare apps within the docs sections for the respective tools, eg: dev client and updates shouldn't delegate to a specific section for this. Instead, the docs within this "Existing React Native apps" section should explain why you would want to use the tools in that context and tell people anything they might need to know beyond what is explained in the normal bare setup instructions (that we then link to).
• Cleaned up a bunch of the docs within the "Existing React Native apps" section.
• Moved the guide about working around running your app in Expo Go even if it has custom native code. We don't want to encourage this, folks should use dev builds.
• Add "Existing native apps" section. This includes our first pass at explaining how to use Expo modules in a "brownfield" app context. I expect we will extend this section more, for now it's more of a stake in the ground with appropriate warnings.

[expo-bot]

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[ide]

In addition to the inline comments, I think it would be good to order Kotlin before Swift since we typically order Android before iOS. That way the programming languages line up (but also K does come before S, but that's not the motivation).

[ide]

Suggestion: move this whole section above "Configure your iOS app" to match the (Android, iOS) ordering we use elsewhere mostly.

[ide]

Explain why you'd want to add it. The phrasing ("you can") says that you have the choice but doesn't explain why you'd want to make that choice or how to decide.