[INFERENCE PROVIDERS] function calling tutorial (#1832)

* first draft of function calling tutorial

* split hf client and openai for complex tool choice mode

* add guide to toc

* drop pointless hfoption

* remove excess hfoption markup

* Apply suggestions from text review

Co-authored-by: Sergio Paniego Blanco <sergiopaniegoblanco@gmail.com>
Co-authored-by: Pedro Cuenca <pedro@huggingface.co>

* clarify provider selection based on feedback

---------

Co-authored-by: Sergio Paniego Blanco <sergiopaniegoblanco@gmail.com>
Co-authored-by: Pedro Cuenca <pedro@huggingface.co>

Author: 3 people

docs/inference-providers/_toctree.yml

@@ -50,6 +50,8 @@
     title: Building Your First AI App
   - local: guides/structured-output
     title: Structured Outputs with LLMs
+  - local: guides/function-calling
+    title: Function Calling
 
 - title: API Reference
   sections: