[Lane 5 · docs] GroupNormalization — document groupNorm in the NN DSL docs #1382

Sub-issue of #1379 ([Feature]: GroupNormalization forward pass (groupNorm {} DSL currently crashes)).

Lane: 5 · Docs/DARC
Skill needed: AsciiDoc. No Kotlin needed beyond copying an example.
Size: xs (under 1 hour)
Blocked by: #1381

What to do

1. Find where layerNorm/batchNorm are documented under docs/modules/ROOT/pages/ (search for layerNorm).
2. Add a groupNorm entry with a short example and a sentence on when to prefer it over BatchNorm (small batches), citing Wu & He, Group Normalization (arXiv:1803.08494).

Acceptance

• Example compiles against the merged Lane-2 code
• Citation resolves
• Result reported back on the parent issue

Notes

If something here turns out to be wrong or much bigger than the size label, comment on this issue or open a specific bug instead of blocking the parent.

Coverage matrix: docs/modules/ROOT/pages/reference/ops-status-matrix.adoc · How lanes work: Issue taxonomy