[docs] Custom model doc update

[MKhalusova]

This PR adds a documentation update to address the feedback left here, quote:

The clarity of the transformers documentation on custom models could be improved by describing 
the syntax of auto_map in config.json. We never used the register functions and would just directly 
modify the config.json. The behaviour that allowed one to use code on the Hub from another repo 
using "--" doesn't seem to be documented anywhere but we figured out that it was possible because 
save_pretrained saves in this format when using a custom model. The feature does seem to be pretty 
new though (I believe ~6 months ago?) so maybe that is why it hasn't been too well documented yet. 
But we think that if it was better communicated to users that it was possible to do this, more people 
would develop on the Hub as we did.

[HuggingFaceDocBuilderDev]

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

[amyeroberts]

Thanks for adding this!