docs: add notations to special tracks

Author: Danielku15

docs/showcase/special-tracks.mdx

@@ -9,6 +9,16 @@ AlphaTab uses the same drum tab notation as Guitar Pro 5. Since the guitar tab s
 percussion track, it's hidden in case a drum tab is displayed.
 <AlphaTab file="/files/features/Drums.gp5" />
 
+## Numbered musical notation (jiǎnpǔ) <span class="badge badge--info badge--header">since 1.4.0</span>
+A notation system mainly used in Asian countries like China. 
+
+<AlphaTab file="/files/features/numbered.gp" />
+
+## Slash notation <span class="badge badge--info badge--header">since 1.4.0</span>
+A notation system focusing on rhythmic aspects.
+
+<AlphaTab file="/files/features/slash.gp" />
+
 ## Grand Staff
 With Guitar Pro 6 files also grand staffs can be rendered.
 <AlphaTab file="/files/features/Piano.gpx" />