docs: add YAML front matter to the SLI API reference docs (#61)

The three docs/api_reference/trellis-api-sli*.md files used bold-markdown headers
instead of the YAML front matter (package / namespaces / types / version /
last_verified / audience: [llm]) that the Trellis framework reference docs use.
Tooling or LLMs that key off audience:[llm] or the types metadata skipped or
misclassified these docs relative to the rest of the Trellis reference set.

Prepend the standard front matter to all three (additive -- the human-readable
**Package** / **Namespace** / **Purpose** lines stay). The docs are auto-packed
into each package's trellis/ folder by Directory.Build.targets, so republished
packages carry the metadata for every downstream consumer (framework, templates,
future consumers).

Verified by packing Trellis.ServiceLevelIndicators: the trellis/trellis-api-sli.md
entry now begins with the front matter block.

Author: xavierjohn

docs/api_reference/trellis-api-sli-apiversioning.md

@@ -1,3 +1,12 @@
+---
+package: Trellis.ServiceLevelIndicators.Asp.ApiVersioning
+namespaces: [Trellis.ServiceLevelIndicators]
+types: [ServiceLevelIndicatorServiceCollectionExtensions, ApiVersionEnrichment, AddApiVersion]
+version: v10
+last_verified: 2026-06-18
+audience: [llm]
+---
+
 # Trellis.ServiceLevelIndicators.Asp.ApiVersioning — API Reference
 
 **Package:** `Trellis.ServiceLevelIndicators.Asp.ApiVersioning`  

docs/api_reference/trellis-api-sli-asp.md

@@ -1,3 +1,12 @@
+---
+package: Trellis.ServiceLevelIndicators.Asp
+namespaces: [Trellis.ServiceLevelIndicators]
+types: [IServiceLevelIndicatorBuilder, ServiceLevelIndicatorCoreServiceCollectionExtensions, ServiceLevelIndicatorServiceCollectionExtensions, ServiceLevelIndicatorApplicationBuilderExtensions, EndpointBuilderExtensions, ServiceLevelIndicatorAttribute, CustomerResourceIdAttribute, MeasureAttribute, CustomerResourceIdMetadata, MeasureMetadata, WebEnrichmentContext, IServiceLevelIndicatorFeature, HttpContextExtensions]
+version: v10
+last_verified: 2026-06-18
+audience: [llm]
+---
+
 # Trellis.ServiceLevelIndicators.Asp — API Reference
 
 **Package:** `Trellis.ServiceLevelIndicators.Asp`  

docs/api_reference/trellis-api-sli.md

@@ -1,3 +1,12 @@
+---
+package: Trellis.ServiceLevelIndicators
+namespaces: [Trellis.ServiceLevelIndicators]
+types: [ServiceLevelIndicator, ServiceLevelIndicatorOptions, MeasuredOperation, SliOutcome, IEnrichmentContext, "IEnrichment<T>", IServiceLevelIndicatorBuilder, ServiceLevelIndicatorMeterProviderBuilderExtensions, AddServiceLevelIndicator, AddServiceLevelIndicatorInstrumentation]
+version: v10
+last_verified: 2026-06-18
+audience: [llm]
+---
+
 # Trellis.ServiceLevelIndicators — API Reference
 
 **Package:** `Trellis.ServiceLevelIndicators`