Revert "remove example gradient-circle"

This reverts commit fef7a99.

Author: hengkx

examples/gradient-circle.html

@@ -0,0 +1 @@
+placeholder

examples/gradient-circle.js

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+import 'rc-progress/assets/index.less';
+import React, { Component } from 'react';
+import ReactDOM from 'react-dom';
+import { Circle } from 'rc-progress';
+
+class Example extends Component {
+  render() {
+    const circleContainerStyle = {
+      width: '250px',
+      height: '250px',
+      display: 'inline-block',
+    };
+    return (
+      <div>
+        <h3>Circle Progress {90}%</h3>
+        <div style={circleContainerStyle}>
+          <Circle
+            percent={90}
+            strokeWidth="6"
+            strokeLinecap="round"
+            strokeColor={{
+              '0%': '#108ee9',
+              '100%': '#87d068',
+            }}
+          />
+        </div>
+        <h3>Circle Progress {100}%</h3>
+        <div style={circleContainerStyle}>
+          <Circle
+            percent={100}
+            strokeWidth="6"
+            strokeLinecap="round"
+            strokeColor={{
+              '100%': '#87d068',
+              '0%': '#108ee9',
+            }}
+          />
+        </div>
+      </div>
+    );
+  }
+}
+
+ReactDOM.render(<Example />, document.getElementById('__react-content'));