Playground: Add help documentation

Base/usr/share/man/man1/Playground.md

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+## Name
+
+Playground - GUI Markup Language (GML) editor
+
+## Synopsis
+
+```**sh
+$ Playground
+```
+
+## Description
+
+Playground facilitates development of graphical user interfaces (GUI)
+for Serenity applications using GUI Markup Language (GML) to compose
+a layout for GUI widgets and set widget attributes.
+
+The specified widgets are automatically rendered in a live preview
+window, allowing rapid prototyping and development of application GUIs.
+
+## Examples
+
+```sh
+$ Playground
+```
+
+## See also
+
+* [`gml-format`(1)](../man1/gml-format.md) For automated GML formatting

Base/usr/share/man/man1/gml-format.md

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+## Name
+
+gml-format - automated GUI Markup Language (GML) formatter
+
+## Synopsis
+
+```**sh
+$ gml-format [--inplace] [path...]
+```
+
+## Description
+
+`gml-format` formats GUI Markup Language (GML) files.
+
+## Options
+
+* `-i`, `--inplace`: Write formatted contents back to file rather than standard output
+
+## Examples
+
+```sh
+$ gml-format -i /home/anon/example.gml
+```