[Flan-UL2] Add-flan-ul2
# What does this PR do?
Adds the documentation for the FLan-UL2 model
cc @younesbelkada 
Fixes #21917
Also, why are the international README entries in the wrong language?<|||||>This should not be the case! Will fix this. <|||||>@sgugger it seems that the README translations are not correct for several languages (korean, jp, etc) due to the fact that they do not follow the correct structure ("released with the repository", "released with the blogpost", ..), As this requires a bit of work I think that we can address a solution in a follow-up PR! Wdyt?<|||||>Btw, I opened a new issue for the README translations to keep things nice and organized (#21932).<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks a lot! 