Add TinyStories to the pretraining docs

As far as I know, one can pretrain fine using TinyStories:

litgpt/pretrain.py --data litgpt.data.TinyStories 

Should we add this to the documentation?

Right now, we only have a pretrain_tinyllama.md doc that pretrains on SlimPajama and StarCoder.

What I propose is

• create a general pretraining.md
• inside this document
  • explain the general pretraining script
  • add the pretrain_tinyllama.md contents as a section there
  • add a TinyStories section

What do you think?