feat: [Unreal] Add transaction and span set_data documentation (#14599)

Preview:
https://sentry-docs-git-unreal-attach-data.sentry.dev/platforms/unreal/tracing/instrumentation/custom-instrumentation/

## DESCRIBE YOUR PR
* Add transaction and span set_data documentation
* Relates to getsentry/projects#524

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Author: mujacica

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

@@ -14,6 +14,8 @@ To capture transactions and spans customized to your organization's needs, you m
 
 <PlatformContent includePath="performance/add-spans-example" />
 
+<PlatformContent includePath="performance/improving-data" />
+
 ## Distributed Tracing
 
 In order to use distributed tracing, follow the <PlatformLink to="/tracing/trace-propagation/custom-instrumentation/">custom instrumentation</PlatformLink> steps.

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

@@ -0,0 +1,35 @@
+## 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 and maps of these types:
+
+```cpp
+USentryTransaction* transaction = SentrySubsystem->StartTransaction(TEXT("Automation transaction"), TEXT("Automation operation"));
+
+transaction->SetData(TEXT("key1"), {{TEXT("data_key1"), TEXT("value")}});
+transaction->SetData(TEXT("key2"), {{TEXT("data_key2"), 123}});
+transaction->SetData(TEXT("key3"), {{TEXT("data_key3"), 123.456f}});
+transaction->SetData(TEXT("key4"), {{TEXT("data_key4"), TArray<FSentryVariant>{{123, 456}}}});
+transaction->SetData(TEXT("key5"), {{TEXT("data_key5"), TMap<FString, FSentryVariant>{{TEXT("inner_key1"), 123}}}});
+
+transaction->Finish();
+```
+
+### 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 and maps of these types:
+
+```cpp
+USentryTransaction* transaction = SentrySubsystem->StartTransaction(TEXT("Automation transaction"), TEXT("Automation operation"));
+USentrySpan* span = transaction->StartChildSpan(TEXT("Automation span"), TEXT("Description text"));
+
+span->SetData(TEXT("key1"), {{TEXT("data_key1"), TEXT("value")}});
+span->SetData(TEXT("key2"), {{TEXT("data_key2"), 123}});
+span->SetData(TEXT("key3"), {{TEXT("data_key3"), 123.456f}});
+span->SetData(TEXT("key4"), {{TEXT("data_key4"), TArray<FSentryVariant>{{123, 456}}}});
+span->SetData(TEXT("key5"), {{TEXT("data_key5"), TMap<FString, FSentryVariant>{{TEXT("inner_key1"), 123}}}});
+
+span->Finish();
+transaction->Finish();
+```