Add error check to detect_clearsky

[cwhanse]

• Closes More descriptive errors for detect_clearsky #2005
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

Adds a check on window_length in clearsky.detect_clearsky to ensure window_length contains at least 3 data points.

[cwhanse]

Adding a bit of scope here. #1784 added a function to lookup thresholds from Jordan and Hansen (2023). That paper extended the detect_clearsky algorithm to plane-of-array (POA) data. #1784 didn't update the docstring to describe the new capability.

[kandersolar]

Thanks @cwhanse!