Update PWA docs to point at the cra-template-pwa package

[slieschke]

The links in the Progressive Web App docs page refer to some files which used to live inside cra-template package but now exist in the cra-template-pacakge in the cra-template/pwa repo. I've updated the template links to point to the new location, as well as some minor edits to account for serviceWorker.js being renamed to serviceWorkerRegistration.js.

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[ianschmitz]

This is great. Thanks @slieschke!