Add documentation URL to API v0 WARN message

[joshpuetz]

A suggestion from @pkfrank:

Curious: would it make sense to add a link to a URL with more information for v0 API users who see the 299 notice?
I’m thinking about my experience as a V0 API user and saying: “Okay, tell me more.”

We should change the current v0 API WARN message to include a link to the documentation about accept header and api-key requirements once that documentation is posted via forem/forem-docs#87

[github-actions]

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[mirie]

Added Blocked label and moved to Next status.