(DOCSP-15227) Add purchasing and billing pages (#12)

* (DOCSP-15227) Initial billing page, purchase and activate tutorial

Author: zach-carr

source/billing.txt

@@ -0,0 +1,25 @@
+.. _billing:
+
+=======
+Billing
+=======
+
+.. default-domain:: mongodb
+
+.. contents:: On this page
+   :local:
+   :backlinks: none
+   :depth: 2
+   :class: singlecol
+
+You can purchase a |servicegov| subscription from the following 
+sources:
+
+- The AWS marketplace
+
+- :website:`MongoDB Sales </contact/atlas>`
+
+Limitations
+-----------
+
+You cannot use a credit card or PayPal to pay for |servicegov-short|.

source/getting-started.txt

@@ -28,6 +28,7 @@ This tutorial guides you through creating:
 .. toctree::
    :titlesonly:
 
+   /tutorial/purchase-subscription
    /tutorial/create-account
    /tutorial/create-organization
    /tutorial/create-project

source/index.txt

@@ -54,4 +54,5 @@ Links to |service| documentation pages are presented as either:
    /atlas-access
    /backup-restore-data
    /api
+   /billing
    /support

source/introduction.txt

@@ -12,4 +12,3 @@
    /introduction/supported-features
    /introduction/atlasgov-considerations
    /introduction/supported-regions
-   /introduction/billing

source/introduction/billing.txt

@@ -0,0 +1,28 @@
+.. _intro-purchase-subscription:
+
+==========================================
+Purchase a |servicegov-short| Subscription
+==========================================
+
+.. default-domain:: mongodb
+
+.. contents:: On this page
+   :local:
+   :backlinks: none
+   :depth: 1
+   :class: singlecol
+
+You can purchase a |servicegov| subscription from the following 
+sources:
+
+- The AWS marketplace
+
+- :website:`MongoDB Sales </contact/atlas>`
+
+Activate Your Subscription
+--------------------------
+
+Placeholder for procedure.
+
+To learn more about |servicegov-short| subscriptions, see 
+:ref:`billing`.