Next.js: [Examples] Add example for Progressive Web Apps

[lfades]

We currently don't mention PWAs in any part of our docs, and PWAs have been gaining popularity. An example called progressive-web-app could be quite useful.

I'm not sure about how PWAs are currently build with Next.js, but next-pwa seems to be doing a good job. The example should feature static pages, without a custom server.

Once the example is done, add it to our FAQ page!

[khattakdev]

I can take this one