admin: 整理了 administrator-guide.xml

* 缩进对齐，注释掉没有完善或者近期不准备完善的章节
* 加入 TODO
* 修改大 part 的标题及说明

Signed-off-by: May <sirhuntthepure@users.noreply.github.com>
admin/clusters: add missing picture

Signed-off-by: May <sirhuntthepure@users.noreply.github.com>
admin/clusters: use xref instead of link

Signed-off-by: May <sirhuntthepure@users.noreply.github.com>