Define technical documentation guidance and template (#50332)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50332

Changelog: [internal]

This defines a detailed strategy on how to approach internal technical documentation in React Native, including defining a template for subsystem documentation.

Reviewed By: javache, rshest

Differential Revision: D71889754

fbshipit-source-id: 5ce630b2e71f8c7b6aaf027a5833bcbdf0dfa5de