readded hugo-book theme files.

themes/hugo-book/exampleSite/content.en/docs/shortcodes/buttons.md

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+# Buttons
+
+Buttons are styled links that can lead to local page or external link.
+
+## Example
+
+```tpl
+{{</* button relref="/" [class="..."] */>}}Get Home{{</* /button */>}}
+{{</* button href="https://github.com/alex-shpak/hugo-book" */>}}Contribute{{</* /button */>}}
+```
+
+{{<button href="/">}}Get Home{{</button>}}
+{{<button href="https://github.com/alex-shpak/hugo-book">}}Contribute{{</button>}}