Constraints of mdspan policy layout_stride

Wtith great support of Mark Hoemmen and Christian Robert Trott (two authors of std::mdspan), I just finished the mdspan chapter of "C++23 - The Complete Guide" (https://www.cppstd23.com/). I learned something I was not aware of and what is not obvious:

If you specify a layout\_stride mapping, there are several constraints you have to take into account. Otherwise, the resulting code has undefined behavior.

The constaintes are:

* Each layout mapping must be ***unique***. This means that each underlying element may only be reached by one combination of indices. In other words: subsets of this layout may not overlap and iterating over all elements may not visit any element twice.
* Only ***positive offsets*** from one stride to the other are allowed. This, for example, means that you cannot use this layout for direct reverse iterations.
* There must be one way to perform a multi-dimensional iteration so that the ***resulting offsets*** to the underlying memory are **ascending.**

For example:

* [https://www.cppstd23.com/code/mdspan/mdspanstride2.cpp.html](https://www.cppstd23.com/code/mdspan/mdspanstride2.cpp.html) is valid.
* [https://www.cppstd23.com/code/mdspan/mdspanstrideUB4.cpp.html](https://www.cppstd23.com/code/mdspan/mdspanstrideUB4.cpp.html) has undefined behavior.

Hope this helps.

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