Migrated documentation to hugo (#21)

I migrated the documentation from jekyll to hugo.

I used the book theme for hugo, and created a custom shortcode for html code examples. The syntax is the following:

{{< example >}}
<!-- Some HTML code here. It will be rendered using the GNOME UI
styling only, avoiding conflicts with the built in styling -->
{{< /example >}}

{{< example nocode >}}
<!-- The HTML code here will be rendered without displaying the
source code (this is used in the colors page) -->
{{< /example >}}

Screenshots

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CC: @cwunder Closes #21 (closed)