rename configuration.md to widgets.md (#390)

docs/widgets.md

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+# Configuring your site
+
+## Widgets
+
+Widgets define the data type and interface for entry fields. Netlify CMS comes with several built-in widgets, including:
+
+Widget | UI | Data Type
+--- | --- | ---
+`string` | text input | string
+`text` | text area input | plain text, multiline input
+`number` | text input with `+` and `-` buttons | number
+`markdown` | rich text editor with raw option | markdown-formatted string
+`datetime` | date picker widget | ISO date string
+`image` | file picker widget with drag-and-drop | file path saved as string, image uploaded to media folder
+`hidden` | No UI | Hidden element, typically only useful with a `default` attribute
+`list` | text input | strings separated by commas
+
+We’re always adding new widgets, and you can also [create your own](/docs/extending).