Add referrer query param to studio link

[Jephuff]

Theres a new referrer param to let studio know what the referrer is, setting it to docs for the link to studio.

[apollo-cla]

@Jephuff: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[trevorblades]

Thanks @Jephuff 🌮