Include GitHub Enterprise Server setup instructions in user-facing docs

[borsos-ilo]

Recently, we have completed integration with self-hosted GitHub Enterprise and prepared docs on how to create and connect user's app. Now it's time to expose these docs in user-facing documentation - my team prepared 1st version of the instructions, but they certainly need review & vocab/grammar check from a native speaker: https://github.com/wpengine/headless/blob/master/docs/ghe-integration-user-ready.md

Feel free to modify the instructions in case you find any spelling / grammar errors and let me know if any help before pushing these docs to user-facing documentation is needed.