docs: demonstrate matrix! and vector! macros in construction example

[wilmerdooley]

The matrix construction example now leads with the matrix!, dmatrix!, vector!, and dvector! macros as the preferred way to build small vectors and matrices from their entries, with the existing ::new, from_rows, from_columns, from_row_slice, from_column_slice, from_fn, and from_iterator approaches kept alongside for comparison. The fixed-size example was also extended to show vector! and dvector!, and the variable bindings were renumbered to keep the assert_eq! checks aligned with each construction method.

Resolves #988

Changes

• Promote matrix! to the first construction method in the fixed-size example, with a comment explaining row-by-row, comma/semicolon syntax.
• Add vector! and dvector! examples for fixed- and dynamic-size vectors in the same block.
• Add a dmatrix! example at the top of the dynamically-sized matrix block, again with a short comment.
• Renumber the remaining matrix construction variables (e.g. m1 -> m2, dm -> dm0, etc.) and extend the assert_eq! chains to cover the new entries.