Update developer.google.com docs

[JustinBeckwith]

With the breaking changes in the 1.0 release, it looks like we left some outdated code samples lying around:
#243 (comment)

For example, this has the signature of verifyIdToken wrong:
https://developers.google.com/identity/sign-in/web/backend-auth#send-the-id-token-to-your-server

I'm happy to do GitHub stuff, but once it gets into Google3 I'd very much prefer someone else take this on :)

[JustinBeckwith]

cc @jonparrott

[jmdobry]

If you can send me what the new Node.js sample for https://developers.google.com/identity/sign-in/web/backend-auth#using-a-google-api-client-library should be, I'll update it in the docs.

[JustinBeckwith]

From a process perspective - is there a way to get these samples into source control and tested, similar to what you do with cloudsite? This is going to be a persistent problem otherwise.

[JustinBeckwith]

Quick ping :) @jonparrott @jmdobry

[Boboss74]

Maybe, as a first step, add a warning message about the outdated documentation

[JustinBeckwith]

I submitted a PR that I'm waiting for approval on. I'm closing this one as a dup of #251 since it's about the same doc :)