Make rollbacks runbook the SSOT for rollbacks

Also removes outdated deployer pipeline example image.

general/deploy/gitlab-com-deployer.md

-## Overview
+# Overview
 
-GitLab.com deployments are initiated by GitLab Chatops, this triggers
+GitLab.com deployments are initiated by GitLab Chatops, which triggers
 a new pipeline on the [Deployer](https://ops.gitlab.net/gitlab-com/gl-infra/deployer) project.
 
-To see all available options for deploy run `/chatops run deploy --help`:
+To see all available options for deploy run `/chatops run deploy --help`
 
 ## Creating a new deployment for _upgrading_ GitLab
 
 Deployments are initiated using GitLab Chatops.
-For example, to initiate a deployment of 11.8.0 to **staging**:
+For example, to initiate a deployment of 11.8.0 to **staging** (default):
 
 ```
 /chatops run deploy 11.8.0.ee.0
@@ -17,7 +17,7 @@ For example, to initiate a deployment of 11.8.0 to **staging**:
 ## Override variables
 
 Variables can be set on the deployer pipeline to change its behavior or
-the jobs that will be executed.  See [variables.md](../../runbooks/variables.md)
+the jobs that will be executed. See [variables.md](../../runbooks/variables.md).
 
 ### Bypassing Failures
 
@@ -63,28 +63,13 @@ the forced failure on the prepare job by using the option
 This essentially sends the variable `PRECHECK_IGNORE_ERRORS` to the deployment
 pipeline.
 
-### Creating a new deployment for _rolling back_ GitLab
+### Creating a new deployment for _rolling back_ GitLab to an earlier version
 
-**IMPORTANT: Before initiating a rollback ensure to follow the [appropriate runbook](../../runbooks/rollback-a-deployment.md),
-the following paragraph covers how to start a manual rollback pipeline on the deployer project in case of a chatops failure.**
+Rollbacks are covered in detail in the [rollback runbook](../../runbooks/rollback-a-deployment.md).
 
-In case of need it's possible to start a deployer pipeline by initiating a new pipeline
-on the [deployer project](https://ops.gitlab.net/gitlab-com/gl-infra/deployer/-/pipelines/new)
-**default branch** and setting the environment and version CI variables.
+## CI/CD Pipeline for a Deployer
 
-* `DEPLOY_ENVIRONMENT`: **required** (i.e.: `gstg`, `gprd`)
-* `DEPLOY_VERSION`: **required**
-* `DEPLOY_ROLLBACK`: **required for rollback**, set to `true` to initiate a rollback pipeline
-
-_Note that all pipelines initiated on branches will automatically set the
-`--dry-run` flag which can be useful for testing._
-
-![Deployer Pipeline](../images/deployer-example.png)
-
-
-### CICD Pipeline for a Deployer
-
-#### Assets
+### Assets
 
 Assets are either extracted from the assets docker image if it is available or pulled from the omnibus package.
 This is done with a [shell script](https://gitlab.com/gitlab-com/gl-infra/deploy-tooling/-/blob/master/bin/fetch-and-upload-assets) in the deploy pipeline at the same time when we run database migrations in a job called `<ENV>-assets.  See [.gitlab-ci](https://gitlab.com/gitlab-com/gl-infra/deployer/-/blob/d0b25b4c783d6010a9d7d0ce64ae49cdda813bb6/.gitlab-ci.yml#L345-359).