[GUIDES] Improve inference providers documentation with guides (#1797)

* add more detail to the billing page

* add first basic guide on inference

* update index with tts providers

* add building your first app guide

* update toc with guides pages

* move pricing change to separate pr 1799

* add js implementation to first app

* use auto not together

* use auto in full app code

* add app screenshots

* use auto in first api call

* simplify js logic

* Update docs/inference-providers/guides/building-first-app.md

* Apply suggestions from code review

Co-authored-by: célina <hanouticelina@gmail.com>

* add section on specify provider vs auto

---------

Co-authored-by: célina <hanouticelina@gmail.com>

Author: burtenshaw

docs/inference-providers/_toctree.yml

@@ -11,6 +11,13 @@
   - local: security
     title: Security
 
+- title: Guides
+  sections:
+  - local: guides/first-api-call
+    title: Your First API Call
+  - local: guides/building-first-app
+    title: Building Your First AI App
+
 - title: Providers
   sections:
   - local: providers/cerebras