Refactor the browser performance testing docs

From my experience in setting this up
https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/231/diffs

doc/ci/examples/browser_performance.md

 # Browser Performance Testing with the Sitespeed.io container
 
-This example shows how to run the [Sitespeed.io container](https://hub.docker.com/r/sitespeedio/sitespeed.io/) on your code by using
-GitLab CI/CD and [Sitespeed.io](https://www.sitespeed.io) using Docker-in-Docker.
+This example shows how to run the
+[Sitespeed.io container](https://hub.docker.com/r/sitespeedio/sitespeed.io/) on
+your code by using GitLab CI/CD and [Sitespeed.io](https://www.sitespeed.io)
+using Docker-in-Docker.
 
-First, you need a GitLab Runner with the [docker-in-docker executor](../docker/using_docker_build.md#use-docker-in-docker-executor).
-
-Once you set up the Runner, add a new job to `.gitlab-ci.yml`, called `performance`:
+First, you need a GitLab Runner with the
+[docker-in-docker executor](../docker/using_docker_build.md#use-docker-in-docker-executor).
+Once you set up the Runner, add a new job to `.gitlab-ci.yml`, called
+`performance`:
 
 ```yaml
+performance:
   stage: performance
   image: docker:git
+  variables:
+    URL: https://example.com
   services:
     - docker:dind
   script:
     - mkdir gitlab-exporter
-    - wget -O ./gitlab-exporter/index.js https://gitlab.com/gitlab-org/gl-performance/raw/10-5/index.js
+    - wget -O ./gitlab-exporter/index.js https://gitlab.com/gitlab-org/gl-performance/raw/master/index.js
     - mkdir sitespeed-results
-    - docker run --shm-size=1g --rm -v "$(pwd)":/sitespeed.io sitespeedio/sitespeed.io:6.3.1 --plugins.add ./gitlab-exporter --outputFolder sitespeed-results https://my.website.com
+    - docker run --shm-size=1g --rm -v "$(pwd)":/sitespeed.io sitespeedio/sitespeed.io:6.3.1 --plugins.add ./gitlab-exporter --outputFolder sitespeed-results $URL
     - mv sitespeed-results/data/performance.json performance.json
   artifacts:
     paths:
@@ -24,30 +30,77 @@ Once you set up the Runner, add a new job to `.gitlab-ci.yml`, called `performan
     - sitespeed-results/
 ```
 
-This will create a `performance` job in your CI/CD pipeline and will run Sitespeed.io against the webpage you define. The GitLab plugin for Sitespeed.io is downloaded in order to export key metrics to JSON. The full HTML Sitespeed.io report will also be saved as an artifact, and if you have Pages enabled it can be viewed directly in your browser. For further customization options of Sitespeed.io, including the ability to provide a list of URLs to test, please consult their [documentation](https://www.sitespeed.io/documentation/sitespeed.io/configuration/).
+The above example will:
+
+1. Create a `performance` job in your CI/CD pipeline and will run
+   Sitespeed.io against the webpage you defined in `URL`.
+1. The [GitLab plugin](https://gitlab.com/gitlab-org/gl-performance) for
+   Sitespeed.io is downloaded in order to export key metrics to JSON. The full
+   HTML Sitespeed.io report will also be saved as an artifact, and if you have
+   [GitLab Pages](../../user/project/pages/index.md) enabled, it can be viewed
+   directly in your browser.
+
+For further customization options of Sitespeed.io, including the ability to
+provide a list of URLs to test, please consult
+[their documentation](https://www.sitespeed.io/documentation/sitespeed.io/configuration/).
 
-For [GitLab Premium](https://about.gitlab.com/products/) users, key metrics are automatically
-extracted and shown right in the merge request widget. Learn more about [Browser Performance Testing](https://docs.gitlab.com/ee/user/project/merge_requests/browser_performance_testing.html).
+TIP: **Tip:**
+For [GitLab Premium](https://about.gitlab.com/pricing/) users, key metrics are automatically
+extracted and shown right in the merge request widget. Learn more about
+[Browser Performance Testing](https://docs.gitlab.com/ee/user/project/merge_requests/browser_performance_testing.html).
 
 ## Performance testing on Review Apps
 
-The above CI YML is great for testing against static environments, and it can be extended for dynamic environments. There are a few extra steps to take to set this up:
-1. The `performance` job should run after the environment has started.
-1. In the `deploy` job, persist the hostname so it is available to the `performance` job. The same can be done for static environments like staging and production to unify the code path. Saving it as an artifact is as simple as `echo $CI_ENVIRONMENT_URL > environment_url.txt`.
-1. In the `performance` job read the artifact into an environment variable, like `$CI_ENVIRONMENT_URL`, and use it to parameterize the test URL's.
-1. Now you can run the Sitespeed.io container against the desired hostname and paths.
+The above CI YML is great for testing against static environments, and it can
+be extended for dynamic environments. There are a few extra steps to take to
+set this up:
 
-A simple `performance` job would look like:
+1. The `performance` job should run after the dynamic environment has started.
+1. In the `review` job, persist the hostname and upload it as an artifact so
+   it's available to the `performance` job (the same can be done for static
+   environments like staging and production to unify the code path). Saving it
+   as an artifact is as simple as `echo $CI_ENVIRONMENT_URL > environment_url.txt`
+   in your job's `script`.
+1. In the `performance` job, read the previous artifact into an environment
+   variable, like `$CI_ENVIRONMENT_URL`, and use it to parameterize the test
+   URLs.
+1. You can now run the Sitespeed.io container against the desired hostname and
+   paths.
+
+Your `.gitlab-ci.yml` file would look like:
 
 ```yaml
+stages:
+  - deploy
+  - performance
+
+review:
+  stage: deploy
+  environment:
+    name: review/$CI_COMMIT_REF_SLUG
+    url: http://$CI_COMMIT_REF_SLUG.$APPS_DOMAIN
+  script:
+    - run_deploy_script
+    - echo $CI_ENVIRONMENT_URL > environment_url.txt
+  artifacts:
+    paths:
+      - environment_url.txt
+  only:
+    - branches
+  except:
+    - master
+
+performance:
   stage: performance
   image: docker:git
   services:
     - docker:dind
+  dependencies:
+    - review
   script:
     - export CI_ENVIRONMENT_URL=$(cat environment_url.txt)
     - mkdir gitlab-exporter
-    - wget -O ./gitlab-exporter/index.js https://gitlab.com/gitlab-org/gl-performance/raw/10-5/index.js
+    - wget -O ./gitlab-exporter/index.js https://gitlab.com/gitlab-org/gl-performance/raw/master/index.js
     - mkdir sitespeed-results
     - docker run --shm-size=1g --rm -v "$(pwd)":/sitespeed.io sitespeedio/sitespeed.io:6.3.1 --plugins.add ./gitlab-exporter --outputFolder sitespeed-results "$CI_ENVIRONMENT_URL"
     - mv sitespeed-results/data/performance.json performance.json