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.

まず issue #4526 と merge request #4530 を確認し、以前の提案が却下された理由も含めて検討してください。Web manifest と service worker を追加することのトレードオフを評価し、実装を開始する前に、ドキュメントのオフライン利用とインストールで何を提供すべきかを定義してください。