docs/quickstart: update instructions to require bundler

With gem-based themes being bundled in the new site via the `Gemfile`, we
should ask folks to use Bundler wherever possible. This should lead to more
successful installations and getting the base site setup properly.

The only trouble this introduces is it puts a dependency on Bundler. That
said, I'm totally fine with requiring everyone use Bundler for this site.
How could we best install bundler in these instructions?

/cc @jekyll/documentation

site/_docs/quickstart.md

@@ -10,7 +10,8 @@ For the impatient, here's how to get a boilerplate Jekyll site up and running.
 ~ $ gem install jekyll
 ~ $ jekyll new myblog
 ~ $ cd myblog
-~/myblog $ jekyll serve
+~/myblog $ bundle install
+~/myblog $ bundle exec jekyll serve
 # => Now browse to http://localhost:4000
 {% endhighlight %}