Add several pages related to UI.
Move sitemaps, HABPanel to it.
Rename "User Interfaces & Ecosystem" to "Ecosystem".
Rename "Beginner Tutorial" to "Getting Started".
Put the installation guide above the Concepts section.
Also fix https://github.com/openhab/website/issues/270.
Signed-off-by: Yannick Schaus <github@schaus.net>