Add initial draft of runbook to deploy to a specific environment

Part of gitlab-com/gl-infra/delivery#2143

runbooks/deploy-to-a-single-environment.md

+# Deploying to a single environment
+
+This runbook aims to cover the use cases where our auto-deploy pipelines are
+not functioning correctly, or there is a need to deploy to environments out of
+order, and thus you need to manually deploy a specific release to an environment.
+
+## Pre-deploy considerations
+
+* When deploying to a canary environment, make sure canary is unddrained so
+that QA tests are accurately using the canary stage
+
+* Identify the full package version you wish to deploy using XXX
+
+## Deploying to a single environment
+
+* Use chatops to deploy to a single environment
+
+```
+# Staging deploy
+/chatops run deploy 14.X.XXX
+
+# Staging canary deploy
+/chatops run deploy --canary 14.X.XXX
+
+# Production deploy
+/chatops run deploy --production 14.X.XXX
+
+# Production canary deploy
+/chatops run deploy --production --canary 14.X.XXX
+```
+
+## Post-deploy considerations
+
+* If you are deploying to `gstg` or `gprd`, consider the fact that the pipeline
+order expects that these environments are kept very close in sync. If you manually
+deploy to a single environment, you should highly consider manually deploying
+to the other one as well