Added: links from model docs to respective model checkpoints on Hub
To aid navigation and discoverability, this PR adds links to respective model checkpoints on Hub to model docs (one link per model, in a format `https://huggingface.co/models?sort=downloads&search=YOSO`).  
A couple of maintenance changes are also included (copyright update and removal of obsolete disclaimers). 
UPD: reverted copyright to the original date
> Note that we're not changing the copyright years of the files: they are copyrighted from the year they were created.

I can revert them back, but I'd love to know why?<|||||>Because there are many many many files in the library ;-) I also think it is bad practice to remove the year of the creation of the file, so if update there was, it would need to be something like 2020-20203 (for a file created in 2020), but I don't think the update is necessary at all.

@CarlosMFerr might have more insight on this!<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_21388). All of your documentation changes will be reflected on that endpoint.<|||||>Since model summary rework, this PR is no longer relevant. 