update favicon

docs/.vuepress/config.js

@@ -19,6 +19,9 @@ module.exports = {
    * ref：https://v1.vuepress.vuejs.org/config/#head
    */
   head: [
+    ['link', { rel: "apple-touch-icon", sizes: "180x180", href: "/apple-icon.png"}],
+    ['link', { rel: "icon", type: "image/png", sizes: "32x32", href: "/favicon-32x32.png"}],
+    ['link', { rel: "icon", type: "image/png", sizes: "16x16", href: "/favicon-16x16.png"}],
     ['meta', { name: 'theme-color', content: '#3eaf7c' }],
     ['meta', { name: 'apple-mobile-web-app-capable', content: 'yes' }],
     ['meta', { name: 'apple-mobile-web-app-status-bar-style', content: 'black' }]