CI documentation: link to semaphore official documentation is dead

[stopostit]

The link to semaphore documentation leads to a 404: https://doc.scalingo.com/platform/deployment/continuous-integration/deploy-scalingo-from-semaphore-ci

It should be updated

[EtienneM]

There is actually more to update as this documentation page does not seem to match the current SemaphoreCI working. I think the new page to follow is https://docs.semaphoreci.com/using-semaphore/workflows. But I tried to configure SemaphoreCI on one of my repository in order to rewrite this documentation page, and I fail to configure anything. SemaphoreCI never sees my GitHub repository. Hence I unassign myself from this issue.