Links to GenerateSW, InjectManifest, and runtimeCaching config were dead following Workbox docs updates. I believe I updated each link in question to the equivalent information in the new Workbox docs

Fix dead links to Workbox docs in README

In Next 12.2.3 a change was added saying that the `next.config.js` "must NOT have additional properties" and that the `pwa` property was considered as an additional property. Therefore, this commit deletes the `pwa` property from the calculated next config in the default exported function in index.js (aka the `withPWA` function). The returned next config therefore does not have the `pwa` field and the warning does not appear.

PWA doc migrated to web.dev

…2.2.3-warnings

Remove pwa from withPWA return value