Documentation: Better file formats documentation

Base/usr/share/man/man5/pp.md

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+## Name
+
+PP - Pixel Paint application file format (.pp)
+
+## Description
+
+Pixel Paint files store the drawing data produced by the Pixel Paint application.
+
+This is a rough overview of the contents of the files:
+
+- width
+- height
+- layers (optional)
+    - width
+    - height
+    - name
+    - locationx
+    - locationy
+    - opacity_percent
+    - visible
+    - selected
+    - bitmap
+- guides (optional)
+    - offset
+    - orientation
+
+## See also
+
+- [`Userland/Applications/PixelPaint/Image.cpp`](../../../../../Userland/Applications/PixelPaint/Image.cpp)