Add: An introductory guide for text generation
Context: current [text generation doc](https://huggingface.co/docs/transformers/main/en/main_classes/text_generation) is large, difficult to navigate, and may be overwhelming to users who are new to text generation.  

I suggest splitting the document into two parts: 
1. an introductory guide that explains the basics and provides some examples
2. trimmed down API reference to be used for looking up specific parameter descriptions (https://github.com/huggingface/transformers/pull/21112)
 
This PR is the first part. It adds a guide that introduces readers to the text generation strategies, explains the generation configuration defaults, and provides some examples. The second part will follow in a separate PR.


 

_The documentation is not available anymore as the PR was closed or merged._<|||||>Hi! Thank you all for the invaluable feedback! I have addressed all the comments and suggestions. Please let me know if there's anything else I need to improve or if we can merge this PR and https://github.com/huggingface/transformers/pull/21112. <|||||>I think both can be merged. Thank you @MKhalusova ❤️ 