Merge branch 'document-environments' into 'master'

Document environments and deployments

## What does this MR do?

Adds an introduction to environments and deployments and tweaks the `environments` entry in `.gitlab-ci.yml` documentation.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

## What are the relevant issue numbers?

Closes #18683.

## Screenshots (if relevant)

![image](/uploads/1e3355c543abaf7b358f3c37596b1f0b/image.png)

![image](/uploads/58a51505674bcf605039db46833a8ae6/image.png)

/cc @ayufan @axil 

See merge request !4725

doc/ci/README.md

@@ -5,6 +5,7 @@
 - [Get started with GitLab CI](quick_start/README.md)
 - [CI examples for various languages](examples/README.md)
 - [Learn how to enable or disable GitLab CI](enable_or_disable_ci.md)
+- [Environments and deployments](environments.md)
 - [Learn how `.gitlab-ci.yml` works](yaml/README.md)
 - [Configure a Runner, the application that runs your builds](runners/README.md)
 - [Use Docker images with GitLab Runner](docker/using_docker_images.md)

doc/ci/environments.md

+# Introduction to environments and deployments
+
+>**Note:**
+Introduced in GitLab 8.9.
+
+## Environments
+
+Environments are places where code gets deployed, such as staging or production.
+CI/CD [Pipelines] usually have one or more [jobs] that deploy to an environment.
+Defining environments in a project's `.gitlab-ci.yml` lets developers track
+[deployments] to these environments.
+
+## Deployments
+
+Deployments are created when [jobs] deploy versions of code to [environments].
+
+## Defining environments
+
+You can create and delete environments manually in the web interface, but we
+recommend that you define your environments in `.gitlab-ci.yml` first, which
+will automatically create environments for you after the first deploy.
+
+The `environment` is just a hint for GitLab that this job actually deploys to
+this environment. Each time the job succeeds, a deployment is recorded,
+remembering the git SHA and environment.
+
+Add something like this to your `.gitlab-ci.yml`:
+```
+production:
+  stage: deploy
+  script: dpl...
+  environment: production
+```
+
+See full [documentation](yaml/README.md#environment).
+
+## Seeing environment status
+
+You can find the environment list under **Pipelines > Environments** for your
+project. You'll see the git SHA and date of the last deployment to each
+environment defined.
+
+>**Note:**
+Only deploys that happen after your `.gitlab-ci.yml` is properly configured will
+show up in the environments and deployments lists.
+
+## Seeing deployment history
+
+Clicking on an environment will show the history of deployments.
+
+>**Note:**
+Only deploys that happen after your `.gitlab-ci.yml` is properly configured will
+show up in the environments and deployments lists.
+
+[Pipelines]: quick_start/README.md
+[jobs]: yaml/README.md#jobs
+[environments]: #environments
+[deployments]: #deployments

doc/ci/yaml/README.md

@@ -530,14 +530,18 @@ The above script will:
 ### environment
 
 >**Note:**
-Introduced in GitLab v8.9.0.
+Introduced in GitLab 8.9.
 
-`environment` is used to define that job does deployment to specific environment.
-This allows to easily track all deployments to your environments straight from GitLab.
+`environment` is used to define that a job deploys to a specific environment.
+This allows easy tracking of all deployments to your environments straight from
+GitLab.
 
-If `environment` is specified and no environment under that name does exist a new one will be created automatically.
+If `environment` is specified and no environment under that name exists, a new
+one will be created automatically.
 
-The `environment` name must contain only letters, digits, '-' and '_'.
+The `environment` name must contain only letters, digits, '-' and '_'. Common
+names are `qa`, `staging`, and `production`, but you can use whatever name works
+with your workflow.
 
 ---
 
@@ -550,7 +554,8 @@ deploy to production:
   environment: production
 ```
 
-The `deploy to production` job will be marked as doing deployment to `production` environment.
+The `deploy to production` job will be marked as doing deployment to
+`production` environment.
 
 ### artifacts