[Docs] Incorrect links to models in the Summary of the Models page
### Issues with the Summary of the Models page
- Description: 
	- Clicking on any model documentation located in https://huggingface.co/transformers/model_summary.html leads us to an empty page.
- Possible Fix: 
	- I've noticed that the GPT link is https://huggingface.co/model_doc/gpt while it should be https://huggingface.co/transformers/model_doc/gpt.html
	- This fix seems to resolve all other issues in the page.

- Observations:
	- I've tried to clone this repo and fix this issue, but for some reason, when I build my own version of the documentation, it doesn't use the `.../transformers/...` path inside the url leading to a different issue from the current online documentation.

- Examples:
<img width="726" alt="Screen Shot 2020-07-06 at 17 28 50" src="https://user-images.githubusercontent.com/9092284/86648750-20481b80-bfaf-11ea-9d3e-b8656d220fdb.png">
EXPECTED:
<img width="1280" alt="Screen Shot 2020-07-06 at 17 29 17" src="https://user-images.githubusercontent.com/9092284/86648771-250ccf80-bfaf-11ea-8552-dd5ab52e36b9.png">
RESULT:
<img width="742" alt="Screen Shot 2020-07-06 at 17 29 01" src="https://user-images.githubusercontent.com/9092284/86648785-28a05680-bfaf-11ea-9578-e16407f58fb4.png">

Fixed in #5566 