docs: explain physical concept and hardware scope on landing page (#365)

Add a short paragraph to index.md that explains what a MagicMirror
physically is, clarifies that this documentation covers the software
side only, and points newcomers to the community showcase for
hardware build inspiration.

Closes MagicMirrorOrg/MagicMirror#3282

Author: KristjanESPERANTO

index.md

@@ -40,6 +40,15 @@ MagicMirror² focuses on a modular plugin system and uses
 [Electron](https://www.electronjs.org/) as an application wrapper. So no more
 web server or browser installs necessary!
 
+**New to the concept?** A MagicMirror is a regular display placed behind a
+two-way mirror. The screen shines through the glass while everything else stays
+reflective – turning an ordinary mirror into a smart display. This documentation
+covers the **software** side only – for the **hardware** side (mirror, frame, mounting)
+things are highly individual and best explored through the
+[community showcase](https://forum.magicmirror.builders/category/12/show-your-mirror)
+where others share their builds and setups. Have questions or ideas?
+[Join the forum](https://forum.magicmirror.builders/).
+
 <p style="display: flex; gap: 10px;justify-content: center">
   <a
     href="https://forum.magicmirror.builders/topic/728/magicmirror-is-voted-number-1-in-the-magpi-top-50"