Split model selection and evaluation tutorials

[leouieda]

The previous model selection tutorial covered both scoring and automatic
tuning. Separate into two tutorials "Evaluating Performance" and "Model
Selection" so they are both shorter.
Include a section in "Model Selection" about the new SplineCV class
from #185.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and verde/__init__.py.
• Write detailed docstrings for all functions/classes/methods.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.