Document DynamoDB backend billing_mode option (gravitational#28672)

* Document on_demand mode option

* update docs for billing_mode option

* use pay_per_request instead of on_demand

docs/pages/reference/backends.mdx

@@ -482,6 +482,11 @@ teleport:
     # By default, Teleport stores audit events with an AWS TTL of 1 year.
     # This value can be configured as shown below. If set to 0 seconds, TTL is disabled.
     audit_retention_period: 365d
+
+    # Enables either On-Demand or Provisioned billing for the DynamoDB table. Set when Teleport creates the table.
+    # Possible values: "pay_per_request" and "provisioned"
+    # default: "pay_per_request"
+    billing_mode: "pay_per_request"
 ```
 
 - Replace `us-east-1` and `Example_TELEPORT_DYNAMO_TABLE_NAME`
@@ -544,6 +549,11 @@ teleport:
     # auto_scaling is used to optionally enable (and define settings for) auto scaling.
     # default: false
     auto_scaling:  [true|false]
+    # Enables either On-Demand or Provisioned billing for the DynamoDB table. Set when Teleport creates the table.
+    # If billing_mode is set to "pay_per_request", read/write capacity and auto_scaling settings will be ignored.
+    # Possible values: "pay_per_request" and "provisioned"
+    # default: "pay_per_request"
+    billing_mode: ["pay_per_request"|"provisioned"]
     # Minimum/maximum read capacity in units
     read_min_capacity: int
     read_max_capacity: int
@@ -581,6 +591,13 @@ To enable these options you will need to update the IAM Policy for Teleport.
 }
 ```
 
+<Admonition
+  type="warning"
+  title="pay_per_request with autoscaling"
+>
+  If `pay_per_request` is set to true, the settings for read/write capacity and auto scaling will be ignored.
+</Admonition>
+
 ### Configuring AWS FIPS endpoints
 
 This config option applies to [AWS S3](#s3) and [AWS DynamoDB](#dynamodb).