Create a custom model guide
First draft of a guide for how to create a model without using any of the `Auto...` classes to give users a better idea of what's happening behind the automagic. The goal of this guide is to show users alternative methods for creating a model. It also demonstrates how users can instantiate these classes themselves if they want to customize or experiment with the default attributes/parameters loaded from a pretrained model. So in a sense, it is also a guide for creating a custom model.

Some feedback I would appreciate:
- Would adding a graphic showing `configuration -> model <- tokenizer/feature extractor/processor` help show how a model is created?
- Would adding an end-to-end code sample (setting up a custom configuration, model and tokenizer) at the end help tie everything together?
- Are there more details I can add around creating a custom model?
- How is my tone for training a model from scratch? I feel like it might be a bit too harsh (see line 118). 😅 

Thanks and let me know if I'm missing anything!
_The documentation is not available anymore as the PR was closed or merged._