Merge pull request #4297 from abpframework/gterdem/docs_button_groups

Create documentation for Button Groups Tag Helper

docs/en/UI/AspNetCore/Tag-Helpers/Button-groups.md

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+# Button groups
+
+## Introduction
+
+`abp-button-group` is the main container for grouped button elements.
+
+Basic usage:
+
+````csharp
+<abp-button-group>
+    <abp-button button-type="Secondary">Left</abp-button>
+    <abp-button button-type="Secondary">Middle</abp-button>
+    <abp-button button-type="Secondary">Right</abp-button>
+</abp-button-group>
+````
+
+## Demo
+
+See the [button groups demo page](https://bootstrap-taghelpers.abp.io/Components/Button-groups) to see it in action.
+
+## Attributes
+
+### direction
+
+A value indicates the direction of the buttons. Should be one of the following values:
+
+* `Horizontal` (default value)
+* `Vertical`
+
+### size
+
+A value indicates the size of the buttons in the group. Should be one of the following values:
+
+* `Default` (default value)
+* `Small`
+* `Medium`
+* `Large`