RN: Replace `master` with `main` in Documentation

Summary:
Updates documentation in React Native to reference `main` (or `HEAD` for URLs) instead of `master`.

Part of https://github.com/facebook/react-native/issues/31788.

Changelog:
[General][Changed] - Update documentation reference from `master` to `main` or `HEAD`.

Reviewed By: JoshuaGross

Differential Revision: D29717128

fbshipit-source-id: 0b0babd8407c6fd3d0e5431f6eaf976059731d6f