docs: recover FAQ in sidebar (#142)

在 PR #124 中误删了侧边栏的常见问题，现在重新添加回来。
```release-note
docs: recover FAQ in sidebar
```

sidebars.js

@@ -63,6 +63,7 @@ module.exports = {
         "user-guide/plugins",
         "user-guide/users",
         "user-guide/settings",
+        "user-guide/faq"
       ],
     },
     {

versioned_sidebars/version-2.0-sidebars.json

@@ -44,7 +44,8 @@
         "user-guide/themes",
         "user-guide/plugins",
         "user-guide/users",
-        "user-guide/settings"
+        "user-guide/settings",
+        "user-guide/faq"
       ]
     },
     {
@@ -137,4 +138,4 @@
     },
     "about"
   ]
-}
+}