DOCS-3667: Document annual billing feature for subscription billing model

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[edobranov]

Thanks for jumping on this! To clarify a bit, there are only three valid use cases right now:

(1) Monthly, in arrears (the existing behavior):

{
  "billing": {
    "cost_per_month": {
      "per_machine": 10
      // ... and any other billable resources, like "binary_data_upload_bytes"
    },
    "tier_name": "monthly-arrears-tier",
    "in_arrears": true
  }
}

(2) Monthly, upfront (working on this right now in APP-8350):

{
  "billing": {
    "cost_per_month": {
      "per_machine": 10
    },
    "tier_name": "monthly-upfront-tier",
    "in_arrears": false   // or omitted
  }
}

(3) Yearly, upfront (the epic's primary goal):

{
  "billing": {
    "cost_per_year": {
      "per_machine": 10
    },
    "tier_name": "yearly-upfront-tier",
    "in_arrears": false   // or omitted
  }
}

Anything else is invalid. Specifically:

• If in_arrears is false, then you can only specify the per_machine billable resource (it didn't make sense to allow charging for other kinds of usage upfront).
• You cannot specify cost_per_year and in_arrears: true (this was more work for us to implement and didn't have a strong product case).

Lmk if I can help clarify anything else!

[npentrel]

Updated this, please have another look.

My only remaining question is:
Is there sufficient error-checking in the UI? I tested and currently I can set tabular_data_upload_bytes to a positive number and in_arrears: false. There's no error message. Maybe it's not fully deployed to prod, idk. Just want to check