Add nbfmt to GitHub CI
Add nbfmt to GitHub CI

Similar to the docs-l10n version: https://github.com/tensorflow/docs-l10n/blob/master/.github/workflows/ci.yaml,
but doesn't require downloading `nbfmt.py`.

Tried on a number of PRs and seems to WAI: https://github.com/tensorflow/docs-l10n/actions

PiperOrigin-RevId: 304098031

tensorflow/docs,tensorflow/docs,tensorflow/docs