Documentation improvements: Enhance clarity and add explanatory content

[ChrisRackauckas-Claude]

This PR improves the documentation quality for SparseBandedMatrices.jl with focused, practical enhancements.

Changes Made

README.md

• Clarified the setdiagonal! example comment to be more descriptive
• Changed "Set the third diagonal from the bottom to have the values [3, 4, 5]" to "Set a lower diagonal with the values [3.0, 4.0, 5.0]" for better clarity

docs/src/index.md

• Enhanced constructor documentation: Added detailed explanation of the pre-specified diagonals constructor parameters
• Added "Understanding Sparse Banded Matrices" section: New section explaining the difference between sparse and dense banded matrices, and the key use cases
• Improved setdiagonal! examples: Added inline comments explaining the true/false parameters
• Enhanced performance considerations:
  • Added more specific guidance on when to use each construction method
  • Mentioned thread-parallel operations
  • Explained benefits of pre-specifying diagonals
• Added diagonal indexing note: Documented the internal indexing scheme (1 to N+M-1 for an N×M matrix)

Rationale

These improvements focus on helping users understand:

1. What sparse banded matrices are and when to use them
2. How to use the API correctly (especially constructor parameters)
3. Performance best practices
4. The difference between this package and dense banded matrix storage

All changes preserve the original documentation structure and voice while adding clarity where needed.

cc @ChrisRackauckas

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