add stack exchange
Hi ! Merging this one since it's all good :)

However I think it would also be better to actually rename it `the_pile_stack_exchange` to make things clearer and to avoid name collisions in the future. I would like to do the same for `books3` as well.

If you don't mind I'll open a PR to do the renaming 
> If you don't mind I'll open a PR to do the renaming

@lhoestq That will be nice !!

stack exchange is part of EleutherAI/The Pile, but AFAIK, The Pile dataset blend all sub datasets together thus we are not able to use just one of its sub dataset from The Pile data. So I create an independent dataset using The Pile preliminary components.

I also change default `timeout` to 100 seconds instead of 10 seconds, otherwise I keep getting read time out when downloading source data of stack exchange and cc100 dataset.

When I was creating dataset card. I found there is room for creating / editing dataset card. I've made it an issue. #2797

Also I am wondering whether the import of The Pile dataset is actively undertaken (because I may need it recently)? #1675