docs: rds resource usage adds an item to pro subscription

[oddgrd]

[joshua-mo-143]

Is this something we still need to add to the docs?

[oddgrd]

It should be updated to say that you need to go to the console and purchase an RDS subscription first, in your account/billing section. We did disable this feature on headroom, though, but perhaps we should still update it in the old platform docs to avoid confusion in the meantime.