Add documentation for GitHub Actions

Add documentation for GitHub Action on https://cakebuild.net/docs/build-systems/

Start at the build-systems documentation section at https://cakebuild.net/docs/build-systems/ and review how neighboring pages are structured. Add a GitHub Actions documentation page or section there, with enough guidance for users to understand the integration. Done means the documentation is published in the requested location and accurately covers GitHub Actions.

Written by the indexing model from the issue text.