Update _toctree.yml (#1926)

Add "Spaces Custom Domain" to TOC

Author: Michellehbn

docs/hub/_toctree.yml

@@ -300,6 +300,8 @@
     title: Spaces Dev Mode
   - local: spaces-storage
     title: Spaces Persistent Storage
+  - local: spaces-custom-domain
+    title: Spaces Custom Domain
   - local: spaces-mcp-servers
     title: Spaces as MCP servers
   - local: spaces-sdks-gradio