Support PWA for offline documentation usage

Hi there,

Saw that #4526 addressed this, and that the OP actually went ahead and submitted merge request #4530. The merge request was rejected because it was unknown what the tradeoffs of including a manifest and service worker were.

I wanted to maybe restart the conversation on this, I think it'd be beneficial to have the docs be available in their original format for offline consumption. There are obviously work arounds (cloning and serving the site locally, offline docs providers), but both of those have fairly significant drawbacks and aren't as easy as right click installing the page.

首先审阅 issue #4526 和 merge request #4530，包括之前的提案被拒绝的原因。评估添加 web manifest 和 service worker 的利弊，然后在开始实现之前，确定文档离线可用性和安装应提供哪些功能。