Create "make stage" for staging container

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

Makefile

@@ -13,3 +13,6 @@ build-preview: ## Build site with drafts and future posts enabled.
 
 serve: ## Boot the development server.
 	jekyll serve
+
+stage: ## Run the Jekyll staging container.
+	docker run -ti --rm -v "$PWD":/k8sdocs -p 4000:4000 gcr.io/google-samples/k8sdocs:1.1

docs/home/contribute/stage-documentation-changes.md

@@ -43,10 +43,13 @@ for this image.
 1. In the root of your cloned repository, enter this command to start a local
 web server:
 
-        docker run -ti --rm -v "$PWD":/k8sdocs -p 4000:4000 gcr.io/google-samples/k8sdocs:1.1
+       make stage
 
-1. View your staged content at
+   This will run the following command:
-http://localhost:4000.
+
+       docker run -ti --rm -v "$PWD":/k8sdocs -p 4000:4000 gcr.io/google-samples/k8sdocs:1.1
+
+1. View your staged content at `http://localhost:4000`.
 
 ## Staging locally without Docker
 
@@ -56,21 +59,20 @@ http://localhost:4000.
 
 1. Verify that Ruby and RubyGems are installed:
 
-        gem --version
+       gem --version
 
 1. Install the GitHub Pages package, which includes Jekyll:
 
-        gem install github-pages
+       gem install github-pages
 
 1. Clone your fork to your local development machine.
 
 1. In the root of your cloned repository, enter this command to start a local
 web server:
 
-        jekyll serve
+       jekyll serve
 
-1. View your staged content at
+1. View your staged content at `http://localhost:4000`.
-http://localhost:4000.
 
 <i>NOTE: If you do not want Jekyll to interfere with your other globally installed gems, you can use `bundler`:</i>