docs: Fix doc img link error (#14957)

* docs: Fix doc img link error

* fix: Update demo pic to cdn link

examples/docs/en-US/card.md

@@ -79,7 +79,7 @@ Display richer content by adding some configs.
 <el-row>
   <el-col :span="8" v-for="(o, index) in 2" :key="o" :offset="index > 0 ? 2 : 0">
     <el-card :body-style="{ padding: '0px' }">
-      <img src="../../examples/assets/images/hamburger.png" class="image">
+      <img src="https://shadow.elemecdn.com/app/element/hamburger.9cf7b091-55e9-11e9-a976-7f4d0b07eef6.png" class="image">
       <div style="padding: 14px;">
         <span>Yummy hamburger</span>
         <div class="bottom clearfix">

examples/docs/es/card.md

@@ -81,7 +81,7 @@ Muestre un contenido más rico añadiendo algunas configuraciones.
 <el-row>
   <el-col :span="8" v-for="(o, index) in 2" :key="o" :offset="index > 0 ? 2 : 0">
     <el-card :body-style="{ padding: '0px' }">
-      <img src="../../examples/assets/images/hamburger.png" class="image">
+      <img src="https://shadow.elemecdn.com/app/element/hamburger.9cf7b091-55e9-11e9-a976-7f4d0b07eef6.png" class="image">
       <div style="padding: 14px;">
         <span>Yummy hamburger</span>
         <div class="bottom clearfix">

examples/docs/fr-FR/card.md

@@ -80,7 +80,7 @@ Affichez un contenu plus riche grâce à la configuration.
 <el-row>
   <el-col :span="8" v-for="(o, index) in 2" :key="o" :offset="index > 0 ? 2 : 0">
     <el-card :body-style="{ padding: '0px' }">
-      <img src="../../examples/assets/images/hamburger.png" class="image">
+      <img src="https://shadow.elemecdn.com/app/element/hamburger.9cf7b091-55e9-11e9-a976-7f4d0b07eef6.png" class="image">
       <div style="padding: 14px;">
         <span>Yummy hamburger</span>
         <div class="bottom clearfix">

examples/docs/zh-CN/card.md

@@ -80,7 +80,7 @@
 <el-row>
   <el-col :span="8" v-for="(o, index) in 2" :key="o" :offset="index > 0 ? 2 : 0">
     <el-card :body-style="{ padding: '0px' }">
-      <img src="../../examples/assets/images/hamburger.png" class="image">
+      <img src="https://shadow.elemecdn.com/app/element/hamburger.9cf7b091-55e9-11e9-a976-7f4d0b07eef6.png" class="image">
       <div style="padding: 14px;">
         <span>好吃的汉堡</span>
         <div class="bottom clearfix">