Refactoring of the text generate API docs
This is part 2 of the refactoring efforts in the text generation docs. The first part (here - https://github.com/huggingface/transformers/pull/21090) adds an introductory guide with examples. 
The second part of the refactor (this PR) reduces repetitive examples and somewhat trims down the API reference doc. Only documentation is affected by this PR.

The text generation API doc page can be trimmed down even further If we remove the docstrings of individual methods like greedy_search(), contrastive_search(), etc. At the moment in 99% of the cases, one can use generate() directly. If it was 100%, I would remove these from the docs. However, I have kept them in the doc at the moment.  


_The documentation is not available anymore as the PR was closed or merged._