docs: make LeJEPA public-facing (TRN-2158) (#1940)

* docs: make LeJEPA public-facing

* fix: removed irrelevant data

* feat: add LeJEPA Lightning and Lightning Distributed examples

Replace the TODO stubs in pytorch_lightning/lejepa.py and
pytorch_lightning_distributed/lejepa.py with real, runnable examples that
mirror examples/pytorch/lejepa.py. The distributed variant enables
LeJEPALoss(gather_distributed=True) so SIGReg aggregates statistics across
DDP ranks, matching the barlowtwins distributed pattern.

Notebooks regenerated via `make generate-example-notebooks`. The docs
lejepa.rst Lightning tabs now resolve to real code via literalinclude.

* refactor: remove target transform

* fix: regenerated noteboos