[Potential documentation improvement] Github integration

I've taken a look at the available Github integration docs here and it currently contains what looks like a client side codegen and git push to Github type of workflow.

This doesn't quite fit the workflow that I'm going for which is -> iterative development of api spec, iterative push api spec to github and have github automatically perform codegen from the api spec upon api spec update.

Perhaps this is more in the realm of CI/CD than what the current docs offer for Github..anyway I've built a working example here within a Github action: https://github.com/jjssoftware/go-user-api/blob/master/.github/workflows/workflow_actions.yml

I'll happily create a PR on https://github.com/OpenAPITools/openapi-generator/blob/master/docs/integration.md if we think it will be of general use to people.

Start with docs/integration.md and compare its current GitHub workflow with the example in .github/workflows/workflow_actions.yml. Document the iterative API-spec push and GitHub Actions code-generation workflow, ensuring the example link and described steps are clear and useful.