docs: Reformat external provider documentation

docs/source/providers/external/index.md

@@ -0,0 +1,13 @@
+# External Providers
+
+Llama Stack supports external providers that live outside of the main codebase. This allows you to:
+- Create and maintain your own providers independently
+- Share providers with others without contributing to the main codebase
+- Keep provider-specific code separate from the core Llama Stack code
+
+```{toctree}
+:maxdepth: 1
+
+external-providers-list
+external-providers-guide
+```