fixed fit-content layout issue and inner child swap issue

docs/intro.md

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+# PyUIBuilder Documentation
+
+>[!NOTE]
+This is a temporary documentation, the page will be updated as more features are added.
+
+
+## Basics Widgets understanding
+
+Every widget has its own attributes, some of the attributes may be common.
+
+1. **MainWindow:** Every UI needs to have one main window. If you don't have any main window, the output will not be generated. 
+   
+   If you have multiple Main Window you'll be asked to delete one window at the time of code generation.
+
+2. **Layouts:** Every widget that can hold a child widget has three different layouts.
+
+   1. Flex(also known as pack)
+   2. Grid
+   3. Absolute/Place
+   
+   The parents of the child widgets controls the layout. The layout properties such as grid position will be available to the child under the grid-manager/flex-manager section.
+