Improve CI/CD For GitHub doc to serve as thorough/enticing feature overview

Per the Q3 documentation OKR requirements, this will be one of the feature docs that we make "more enticing, replacing the need for their about.gitlab.com feature pages".

Feature page: https://about.gitlab.com/features/github

Doc page: https://docs.gitlab.com/ee/ci/ci_cd_for_external_repos/github_integration.html

See the linked epic for a checklist, and update the description here with it.