add CHANGELOG.md for v0.3.0 release

Author: nathanhubens

CHANGELOG.md

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+# Release notes
+
+<!-- do not remove -->
+
+## 0.3.0
+
+### New Features
+- Migrated to nbdev3 with `pyproject.toml` (PEP 621) replacing `settings.ini`
+- Renamed `analysis` module to `analyze` for verb-based naming consistency
+- Agnostic `Schedule` system: schedules return progress (0→1), callbacks multiply by target
+- Per-layer sparsity targets via dict for `SparsifyCallback` and `Pruner`
+- Sensitivity analysis module (`fasterai.analyze.sensitivity`)
+
+### Enhancements
+- Updated CI workflows to nbdev3 actions (`quarto-ghp3`, `nbdev3-ci`)
+- Expanded PyPI keywords and classifiers
+- Fixed repository URLs and metadata for FasterAI-Labs organization
+