Added three videos to documentation.

Created shortcode for YouTube videos and made the videos responsive in the
stylesheet.

docs/content/overview/introduction.md

@@ -37,6 +37,10 @@ build times and the ability to rebuild whenever a change is made, Hugo
 provides a very fast feedback loop. This is essential when you are
 designing websites, but also very useful when creating content.
 
+## How fast is Hugo?
+
+{{% youtube CdiDYZ51a2o %}}
+
 ## What does Hugo do?
 
 In technical terms, Hugo takes a source directory of Markdown files and

docs/content/overview/quickstart.md

@@ -12,6 +12,8 @@ weight: 10
 
 > _Note: This quickstart depends on features introduced in Hugo v0.11.  If you have an earlier version of Hugo, you will need to [upgrade](/overview/installing/) before proceeding._
 
+{{% youtube w7Ft2ymGmfc %}}
+
 ## Step 1. Install Hugo
 
 Go to [Hugo Releases](https://github.com/spf13/hugo/releases) and download the

docs/content/troubleshooting/overview.md

@@ -20,6 +20,8 @@ Here are some examples:
 * [How do I include an image gallery on my website?](http://discuss.gohugo.io/t/image-gallery/594)
 * ... And a lot more!
 
+{{% youtube c8fJIRNChmU %}}
+
 Indeed, you may find many questions and solutions
 to problems in our [discussion forum](http://discuss.gohugo.io/),
 and you may find the [support](http://discuss.gohugo.io/category/support)