Create basic documentation for Event Loop (#50550)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50550

Changelog: [internal]

This creates the basic documentation for the Event Loop, adapting some of the content of the original RFC: https://github.com/react-native-community/discussions-and-proposals/blob/main/proposals/0744-well-defined-event-loop.md

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Reviewed By: lenaic

Differential Revision: D72632590

fbshipit-source-id: fbe30334a567dc642a4b817abb0c535b633277e7