Add marshaller shape docs to docs with more detailed explanations (#47125)

Co-authored-by: Genevieve Warren <24882762+gewarren@users.noreply.github.com>

Author: jtschuster

docs/navigate/advanced-programming/toc.yml

@@ -357,6 +357,8 @@ items:
       href: ../../standard/native-interop/customize-parameter-marshalling.md
     - name: Source generation
       href: ../../standard/native-interop/custom-marshalling-source-generation.md
+    - name: Custom marshaller shapes
+      href: ../../standard/native-interop/custom-marshaller-shapes.md
     - name: "Tutorial: Use custom marshallers in source generation"
       href: ../../standard/native-interop/tutorial-custom-marshaller.md
   - name: Interop best practices