add jekyll-pwa-plugin (#6533)
add jekyll-pwa-plugin (#6533)

Merge pull request 6533
jaybe-jekyll/jekyll,jekyll/jekyll,fulldecent/jekyll,x-way/jekyll,gencer/jekyll,tillgrallert/jekyll,jekyll/jekyll,ryanshaw/jekyll,jaybe-jekyll/jekyll,Strangehill/jekyll,jekyll/jekyll,gencer/jekyll,fulldecent/jekyll,fulldecent/jekyll,gencer/jekyll,getandpost/jekyll.github.io,ryanshaw/jekyll,tillgrallert/jekyll,Strangehill/jekyll,tillgrallert/jekyll,Strangehill/jekyll,Strangehill/jekyll,ryanshaw/jekyll,ryanshaw/jekyll,chrisfinazzo/jekyll,chrisfinazzo/jekyll,jekyll/jekyll,tillgrallert/jekyll,x-way/jekyll,chrisfinazzo/jekyll,Strangehill/jekyll,yhironaka/yhironaka.github.io,jaybe-jekyll/jekyll,chrisfinazzo/jekyll,getandpost/jekyll.github.io,getandpost/jekyll.github.io,x-way/jekyll,x-way/jekyll,tillgrallert/jekyll,fulldecent/jekyll,gencer/jekyll,gencer/jekyll,jaybe-jekyll/jekyll