[docs] update dead quickstart link on resuing past for GPT2
Thed dead link have been replaced by two links of forward and call methods of the GPT2 class for torch and tensorflow respectively.

# What does this PR do?

This PR replaces the dead link with two new links on showing how to reuse the precomputed attention values.

Fixes #13434

## Before submitting
- [ X] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
> Thanks a lot for fixing!
> 
> Could you just run `make style` on your branch to fix the code quality issue?

Done.<|||||>Thanks again!