speed up loading JS assets

[juzhiyuan]

Actual Behaviour

Hi, we're using https://docusaurus.io/ V2 to build our website, when it's built after running yarn build, it will generate many JS assets under the build/assets/js/ directory, e.g https://github.com/apache/apisix-website/tree/asf-site/assets/js

And when we visit our website, it will first load the runtime~main.xxxx.js file, and it will load all necessary other JS assets asynchronously, but with relative URL like the following screenshot:

Expected Behaviour

https://apisix.apache.org/assets/js/69365.a39fd14a.js.

This URL is correct but I would suggest that we load them from jsDelivr, too. So users in China could visit faster.

[juzhiyuan]

Here has one PR I submitted yesterday, due to some issues I reverted it, you could refer to #670

see #671 (comment)