[troubleshooting] add 2 points of reference to the offline mode
As discussed at https://github.com/huggingface/transformers/issues/11231#issuecomment-818976986 the offline mode doc can be hard to find, so this PR:

- starts a new "troubleshooting" document 
- adds a note xref to `from_pretrained`

Surely we can start populating the new "troubleshooting" document - the idea here is to have common problems with explicit error messages and pointers to solutions.

@LysandreJik, @sgugger  