Add deprecation warning for Rio sample data

[leouieda]

Licensing for the dataset is unclear it's best to remove it. We'll
deprecate first and then remove in a future major release. Add warnings
to the docstrings and sample data example. Replace the Rio data with
Texas wind in the Trend gallery example (it's already used in the trend
estimation tutorial). See #201 for more context.

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.

[santisoler]

Seems good! 🥇