Move Billing error codes Post into a Page nested under /billing/

[brandonmwest]

http://docs.sendgrid.com/docs/billing/billing-error-code/ has some useful information in it for users who are trying to sign up and get an error w/ their payments. Suggest move into a Page from a Post and next under Billing drop down.

If URL changes, let me or Jacob know. there is a Zoura template that references it.