chore: add billing group

docs/blueapi/apis/billing/billinggroup.md

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+# Billing group documentation
+
+Description of values related to Billing group.
+
+## Guidance
+
+Definition of Billing group.
+
+```
+message BillingGroup {
+  string billingInternalId = 1;
+
+  string billingGroupId = 2;
+
+  string billingGroupName = 3;
+
+  string companyName = 7;
+}
+```
+
+### billingInternalId
+
+The following section describes the use of the `billingInternalId`.
+
+This is a unique ID given when creating a billing group. This value cannot be changed.
+
+### billingGroupId
+
+The following section describes the use of the `billingGroupId`.
+
+This is a ID for users to manage billing groups in products(Ripple/Wave[Pro]).
+
+### billingGroupName
+
+The following section describes the use of the `billingGroupName`.
+
+This is a Name for users to manage billing groups in products(Ripple/Wave[Pro]).
+
+### companyName
+
+The following section describes the use of the `companyName`.
+
+This is a Name for users to manage company involved in billing.