Add Built-in Layouts

Built-in Layouts.-.md

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+`App.Layout` serve several different purposes. They provide one or multiple of the following features for contained elements (`Layout`'s and/or `Widget`'s):
+
+- structuring
+- decorating
+
+The following Layouts are provided by `zterm`:
+
+- `VStack`
+- `HStack`
+- `Padding`
+- `Margin`
+- `Framing`
+
+# VStack
+
+Vertical stacking of contained elements.
+
+# HStack
+
+Horizontal stacking of contained elements.
+
+# Padding
+
+Provide absolute padding for the contained single element.
+
+# Margin
+
+Provide relative (percentual) margins for the contained single element.
+
+# Framing
+
+Provide a frame (stylable through configuration parameter) for the contained single element.