docs: add version switcher

docs/docsify-version-plugin.js

@@ -0,0 +1,14 @@
+;(function () {
+  function versionPlugin(hook) {
+    hook.mounted(function () {
+      document
+        .getElementById('version-selector')
+        .addEventListener('change', (e) => {
+          window.location.href = '../' + e.target.value + '/'
+        })
+    })
+  }
+
+  $docsify = $docsify || {}
+  $docsify.plugins = [].concat($docsify.plugins || [], versionPlugin)
+})()