Add documentation for the play_youtube module

documentation/modules/post/multi/manage/play_youtube.md

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+`play_youtube` allows you to open and start playing a YouTube video on a
+compromised host.
+
+## Important Options
+
+**EMBED**
+
+Whether or not to use the `/embed` YouTube URL. The embeded version provides a
+clean interface and will start playing in fullscreen but is not compatible with
+all YouTube videos, for example Rick Astley - Never Gonna Give You Up (VID:
+[`dQw4w9WgXcQ`][1]) is not compatible.
+
+While the non-embeded version has greater compatibility, there is a chance that
+an advertisement may be played before the video. It is recommended to use the
+embeded version when the video is compatible.
+
+**VID**
+
+The video's identifier on YouTube. This is the `v` parameter of the URL.
+
+## See Also
+
+* Meterpreter's `uictl` command in the `stdapi` extension for enabling and
+  disabling the mouse and keyboard.
+
+[1]: https://www.youtube.com/watch?v=dQw4w9WgXcQ