PWA: Check if offline mode works correctly

[RusinovAnton]

The PWA setup is ready and there is a possibility to install the application to the Home screen of a smartphone or in Desktop browsers apps.

Looks like it even works in offline mode - the HTML and JS files must be cached right now. But the last time I've checked it, the markdown docs - they weren't available in the offline even I have looked for them when I had an internet connection. the task is to check whether it works correctly and if not - fix it :)