[Docs] Model_doc structure/clarity improvements (#26876)

* first batch of structure improvements for model_docs

* second batch of structure improvements for model_docs

* more structure improvements for model_docs

* more structure improvements for model_docs

* structure improvements for cv model_docs

* more structural refactoring

* addressed feedback about image processors

docs/source/en/model_doc/bartpho.md

@@ -29,7 +29,9 @@ on a downstream task of Vietnamese text summarization show that in both automati
 outperforms the strong baseline mBART and improves the state-of-the-art. We release BARTpho to facilitate future
 research and applications of generative Vietnamese NLP tasks.*
 
-Example of use:
+This model was contributed by [dqnguyen](https://huggingface.co/dqnguyen). The original code can be found [here](https://github.com/VinAIResearch/BARTpho).
+
+## Usage example
 
 ```python
 >>> import torch
@@ -54,7 +56,7 @@ Example of use:
 >>> features = bartpho(**input_ids)
 ```
 
-Tips:
+## Usage tips
 
 - Following mBART, BARTpho uses the "large" architecture of BART with an additional layer-normalization layer on top of
   both the encoder and decoder. Thus, usage examples in the [documentation of BART](bart), when adapting to use
@@ -79,8 +81,6 @@ Tips:
   Other languages, if employing this pre-trained multilingual SentencePiece model "vocab_file" for subword
   segmentation, can reuse BartphoTokenizer with their own language-specialized "monolingual_vocab_file".
 
-This model was contributed by [dqnguyen](https://huggingface.co/dqnguyen). The original code can be found [here](https://github.com/VinAIResearch/BARTpho).
-
 ## BartphoTokenizer
 
 [[autodoc]] BartphoTokenizer