docs: 添加marker demo (#1423)

dev-demos/features/marker/demos/marker.tsx

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+// @ts-ignore
+import { Popup,PointLayer, Marker,Scene } from '@antv/l7';
+// @ts-ignore
+import { GaodeMap, GaodeMapV2, Mapbox } from '@antv/l7-maps';
+import React, { useEffect } from 'react';
+  
+export default () => {
+    useEffect( () => {
+      const scene = new Scene({
+  id: 'map',
+  map: new GaodeMap({
+    style: 'normal',
+    center: [ 120.104446,30.261081 ],
+    zoom: 19.056
+  })
+});
+scene.on('loaded', () => {
+// 创建默认 marker
+  const popup = new Popup({
+    offsets: [ 0, 20 ]
+  }).setText('Hello World 1024');
+
+  const marker = new Marker()
+    .setLnglat([ 120.1047383116185,30.261127905299425 ])
+    .setPopup(popup);
+    const pointLayer = new PointLayer({})
+    .source([{
+        x:120.1047383116185,
+        y:120.1047383116185
+    }
+    ],{
+        parser: {
+            type: 'json',
+            x: 'x',
+            y: 'y',
+        },
+    })
+    .shape('circle')
+    .size(5)
+    .color('red')
+    .style({
+        opacity: 1,
+        strokeWidth: 0,
+        stroke: '#fff',
+    });
+    scene.on('click',(e)=>{
+        console.log(e);
+    })
+scene.addLayer(pointLayer);
+  scene.addMarker(marker);
+  marker.togglePopup()
+});
+
+          
+    }, []);
+    return (
+      <div
+        id="map"
+        style={{
+          height: '500px',
+          position: 'relative',
+        }}
+      />
+    );
+  };
+  

dev-demos/features/marker/marker.md

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+### Marker - 标注
+<code src="./demos/marker.tsx"></code>