Tweak and move plymouth doc to guides

The plymouth splash screen application is still being used, but the
GStreamer module written for Apertis is not currently enabled in the
package. As the patches for this support are still available comment out
the mentions of this functionality so they may be re-instated if the
support is added back. Move document to guides as it covers how to use and
modify plymouth.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>