Add warning about fixed splits in `ketos train`/`ketos test` #868

Job	Run time
Runs the linter and tests (3.8)	7m 54s
Runs the linter and tests (3.9)	8m 21s
Runs the linter and tests (3.10)	8m 4s
Runs the linter and tests (3.11)	8m 47s
Update kraken.re github pages	1m 2s
Build and publish anaconda packages	0s
Build and publish Python 🐍 distributions 📦 to PyPI and TestPyPI	0s
Create github release	0s
	34m 8s

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