Updates the Unity SDK docs with new SetData methods on spans & transa… (#13457)

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## DESCRIBE YOUR PR
Updates the Unity SDK docs with new SetData methods on spans &
transactions

SDK changes: getsentry/sentry-dotnet#3936

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---------

Co-authored-by: Stefan Pölz <[email protected]>

Author: lukemun

docs/platforms/unity/tracing/instrumentation/custom-instrumentation.mdx

@@ -15,3 +15,5 @@ To capture transactions and spans customized to your organization's needs, you m
 <PlatformContent includePath="performance/add-spans-example" />
 
 <PlatformContent includePath="performance/retrieve-transaction" />
+
+<PlatformContent includePath="performance/improving-data" />

platform-includes/configuration/data-to-all-spans/unity.mdx

@@ -0,0 +1,18 @@
+```csharp
+options =>
+{
+    options.SetBeforeSendTransaction(transaction =>
+    {
+        // Set the attribute on the root span (transaction's trace context)
+        transaction.Contexts.Trace.SetData("myAttribute", "myValue");
+
+        // And on all child spans
+        foreach (var span in transaction.Spans)
+        {
+            span.SetData("myAttribute", "myValue");
+        }
+
+        return transaction;
+    });
+};
+```

platform-includes/performance/improving-data/unity.mdx

@@ -0,0 +1,39 @@
+## Improving Data on Transactions and Spans
+
+### Adding Data Attributes to Transactions
+
+You can add data attributes to your transactions. This data is visible and __queryable__ in the trace explorer in Sentry. Data attributes can be of type _string_, _number_ or _boolean_, as well as arrays of these types:
+
+```csharp
+var transaction = SentrySdk.StartTransaction("ProcessOrderBatch()", "task");
+SentrySdk.ConfigureScope(scope => scope.Transaction = transaction);
+transaction.SetData("my-data-attribute-1", "value1");
+transaction.SetData("my-data-attribute-2", 42);
+transaction.SetData("my-data-attribute-3", true);
+
+transaction.SetData("my-data-attribute-4", new[] {"value1", "value2", "value3"});
+transaction.SetData("my-data-attribute-5", new[] {42, 43, 44});
+transaction.SetData("my-data-attribute-6", new[] {true, false, true});
+```
+
+### Adding Data Attributes to Spans
+
+You can add data attributes to your transactions. This data is visible and __queryable__ in the trace explorer in Sentry. Data attributes can be of type _string_, _number_ or _boolean_, as well as arrays of these types:
+
+
+```csharp
+var span = parent.StartChild("task", "operation");
+span.SetData("my-data-attribute-1", "value1");
+span.SetData("my-data-attribute-2", 42);
+span.SetData("my-data-attribute-3", true);
+
+span.SetData("my-data-attribute-4", new[] {"value1", "value2", "value3"});
+span.SetData("my-data-attribute-5", new[] {42, 43, 44});
+span.SetData("my-data-attribute-6", new[] {true, false, true});
+```
+
+### Adding Attributes to all Spans and Transactions
+
+To add an attribute to all spans and transactions, use the `SetBeforeSendTransaction` callback:
+
+<PlatformContent includePath="configuration/data-to-all-spans" />