[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/bigbird_pegasus.md

@@ -41,7 +41,9 @@ sequence as part of the sparse attention mechanism. The proposed sparse attentio
 BigBird drastically improves performance on various NLP tasks such as question answering and summarization. We also
 propose novel applications to genomics data.*
 
-Tips:
+The original code can be found [here](https://github.com/google-research/bigbird).
+
+## Usage tips
 
 - For an in-detail explanation on how BigBird's attention works, see [this blog post](https://huggingface.co/blog/big-bird).
 - BigBird comes with 2 implementations: **original_full** & **block_sparse**. For the sequence length < 1024, using
@@ -54,9 +56,7 @@ Tips:
 - BigBird is a model with absolute position embeddings so it's usually advised to pad the inputs on the right rather than
   the left.
 
-The original code can be found [here](https://github.com/google-research/bigbird).
-
-## Documentation resources
+## Resources
 
 - [Text classification task guide](../tasks/sequence_classification)
 - [Question answering task guide](../tasks/question_answering)