[docs] Text to speech task guide (#23107)

* First draft

* Some polishing

* Text polishing

* added TOC entry for TTS

* make style

* added links to images

* fixed links to images

* Apply suggestions from code review

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* feedback addressed

* feedback from Matthijs addresed

* Update docs/source/en/tasks/text-to-speech.mdx

Co-authored-by: Matthijs Hollemans <mail@hollance.com>

---------

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Matthijs Hollemans <mail@hollance.com>

docs/source/en/_toctree.yml

@@ -71,6 +71,8 @@
         title: Image captioning
       - local: tasks/document_question_answering
         title: Document Question Answering
+      - local: tasks/text-to-speech
+        title: Text to speech
     title: Multimodal
     isExpanded: false
   title: Task Guides