[docs] IDEFICS guide and task guides restructure (#26035)

* initial commit for the IDEFICS task guide

* conversational example

* updated TOC

* fixed typos

* Apply suggestions from code review
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* addressed feedback

* bad_words_ids

* Apply suggestions from code review
Co-authored-by: Victor SANH <victorsanh@gmail.com>

* rank classification note

* feedback addressed

---------
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
Co-authored-by: Victor SANH <victorsanh@gmail.com>

docs/source/en/_toctree.yml

@@ -88,6 +88,11 @@
     - local: generation_strategies
       title: Customize the generation strategy
     title: Generation
+  - isExpanded: false
+    sections:
+    - local: tasks/idefics
+      title: Image tasks with IDEFICS
+    title: Prompting
   title: Task Guides
 - sections:
   - local: fast_tokenizers