Move image mirror introcution to the installation chapter

Fix issue #136.

SUMMARY.md

@@ -12,6 +12,7 @@
     * [Ubuntu、Debian](install/ubuntu.md)
     * [CentOS](install/centos.md)
     * [macOS](install/mac.md)
+    * [镜像加速器](install/mirror.md)
 * [镜像](image/README.md)
     * [获取镜像](image/pull.md)
     * [列出镜像](image/list.md)
@@ -33,7 +34,6 @@
         * [参考文档](image/dockerfile/references.md)
     * [其它制作镜像的方式](image/other.md)
     * [删除本地镜像](image/rmi.md)
-    * [镜像加速器](image/mirror.md)
     * [实现原理](image/internal.md)
 * [容器](container/README.md)
     * [启动](container/run.md)