Create concept guide section
This PR wraps up the IA migration:

- Created a Conceptual Guide section and migrated docs that are explanatory.
- Created a separate page in Conceptual Guides for this [section](https://huggingface.co/docs/transformers/main/en/preprocessing#everything-you-always-wanted-to-know-about-padding-and-truncation) about padding and truncation from the Preprocess doc.
- Reorganized table of contents to group similar topics together like contributing (how to add model/pipeline/testing etc.). Feel free to suggest any changes :)
- Renamed [Create a custom ~~model~~](https://huggingface.co/docs/transformers/main/en/create_a_model) to `architecture` based on @NielsRogge feedback that this guide only describes how to customize the architecture from the config. Also feel free to suggest another title here, I'm not too excited with what I have right now. The title should be distinct from [Sharing custom models](https://huggingface.co/docs/transformers/main/en/custom_models), which actually shows you how to write your own custom config and model.
- Updated the `index` to reflect how the docs are organized.
- Removed older versions of docs that have been updated ([fine-tune ](https://huggingface.co/docs/transformers/main/en/custom_datasets )and [multilingual](https://huggingface.co/docs/transformers/main/en/multilingual)).
_The documentation is not available anymore as the PR was closed or merged._