docs(nalgebra-sparse): clarify required features for front-page example

[wilmerdooley]

The front-page example in nalgebra-sparse uses matrixcompare to assert that the sparse and dense matrices agree, but the surrounding documentation did not mention the compare feature on nalgebra and nalgebra-sparse that this example needs. Without those features, new users copy-pasting the example will hit unresolved imports, which has been a source of confusion.

This change adds a short note and a Cargo.toml snippet above the example in nalgebra-sparse/src/lib.rs so readers can see exactly which dependencies and features to enable.

Resolves #1214

Changes

• Added a paragraph to the front-page example in nalgebra-sparse/src/lib.rs explaining that the compare feature is required on both nalgebra-sparse and nalgebra.
• Added a Cargo.toml snippet showing the required dependencies and feature flags for the example to compile.