Add themes in-tree

themes/github.com/davegallant/hugo-theme-gruvbox/content/blog/rich-content.md

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+author = "Hugo Authors"
+title = "Rich Content"
+date = "2019-03-10"
+description = "A brief description of Hugo Shortcodes"
+tags = [
+    "shortcodes",
+    "privacy",
+    "hugo-basic-example",
+]
++++
+
+Hugo ships with several [Built-in Shortcodes](https://gohugo.io/content-management/shortcodes/#use-hugos-built-in-shortcodes) for rich content, along with a [Privacy Config](https://gohugo.io/about/hugo-and-gdpr/) and a set of Simple Shortcodes that enable static and no-JS versions of various social media embeds.
+<!--more-->
+---
+
+## YouTube Privacy Enhanced Shortcode
+
+{{< youtube ZJthWmvUzzc >}}
+
+<br>
+
+---
+
+## Twitter Simple Shortcode
+
+{{< twitter_simple user="DesignReviewed" id="1085870671291310081" >}}
+
+<br>
+
+---
+
+## Vimeo Simple Shortcode
+
+{{< vimeo_simple 48912912 >}}