DOC: clarify Linke turbidity source and usage in Ineichen clearsky model

[kumaradityaapril]

• Closes Confusing docs for pvlib.clearsky.ineichen and lookup_linke_turbidity #2598
• I am familiar with the contributing guidelines
• Tests added (not applicable – documentation-only change)
• Updates entries in docs/sphinx/source/reference for API changes (not applicable)
• Adds entries in docs/sphinx/source/whatsnew (not required for docs-only change)
• New code is fully documented (docstrings updated with examples)
• Pull request is nearly complete and ready for detailed review
• Maintainer: Appropriate GitHub Labels and Milestone (maintainer action)

Description

The documentation for pvlib.clearsky.ineichen could be misleading, as it
suggested that default Linke turbidity values from SoDa are used implicitly.
In reality, these values must be explicitly obtained via
lookup_linke_turbidity and passed to ineichen.

This PR clarifies the documentation by:

• Updating the ineichen docstring to explicitly reference
  lookup_linke_turbidity as the source of SoDa Linke turbidity values
• Adding data provenance (SoDa) to the lookup_linke_turbidity docstring
• Adding a short example demonstrating the two-step workflow
  (lookup_linke_turbidity → ineichen)

This is a documentation-only change and does not modify any functionality.