Fix rustdoc warnings

[CGMossa]

Ran cargo doc --workspace --no-deps --document-private-items using

❯ rustdoc --version
rustdoc 1.86.0 (05f9846f8 2025-03-31)

Plus, I applied suggestions from markdownlint.

Additions due to CI

I had to update the rustc version in CI from 1.79 to 1.81 due to a dev-dependency.

❯ cargo tree -i half                       
half v2.6.0
└── ciborium-ll v0.2.2
    └── ciborium v0.2.2
        └── criterion v0.4.0
            [dev-dependencies]
            └── nalgebra v0.33.2 (/Users/elea/Documents/GitHub/nalgebra)
                [dev-dependencies]
                └── nalgebra v0.33.2 (/Users/elea/Documents/GitHub/nalgebra) (*)

Also, now the snapshots must be upgraded using TRYBUILD=overwrite cargo +1.81 ..., as there are serious differences in how latest stable behaves vs. 1.81.