Migrate to VuePress2

[Mister-Hope]

Though it isn't necessary, but it's pretty werid for a "Vue3 CN Docs" website building with Vue2.

I would suggest to migrate to VuePress2 as it's based on Vue3 and has a lot of performance improvements comparing with VuePress1.

If you would like to take the suggestion, I am expecting that you can start the migration yourself.

However I can help to migrate to VuePress2 if you need a pr because of reasons like lacking of spare time, unfamilar with VuePress2 and etc.