Update User Guide with matrix! use

[Andlon]

The User Guide on vectors and matrices should be updated with instructions for how to use the matrix! macro (and related vector!, dvector! and dmatrix! macros) to construct matrices, as this is now the preferred way of constructing small matrices directly based on entries.

See #987 for the issue that prompted this issue.

[anirudhRowjee]

Hi! Can I work on this?

[Andlon]

Hi @anirudhRowjee: that would be great! I don't know of anyone else working on this, so I think it's all yours :-)

Let us know if you have any questions. Some of us are also active on Discord, and you can find some more information, including some notes on how to contribute to the website/user guide, here: https://www.nalgebra.org/community

[lvllvl]

Hi,

I created a PR for this specific issue here.