Update cppreference with mdspan

Hi! I am working with mdspan every now and then, so, naturally, I need to consult its documentation. I usually used P0009 as a reference, but since it was approved in WG21 plenary and merged to the C++ working draft, it has not been updated anymore. Several follow-up papers have been proposed and accepted in the meantime, so there is no longer a single up-to-date reference.

At some point I noticed someone started populating cppreference with mdspan: https://en.cppreference.com/w/cpp/container/mdspan. However, just the synopsis of std::mdspan is given. It would really help me as a user, if you could take a bit of time to add a synopsis for the related classes as well, e.g. for the layouts and accessors. Also, consider adding the individual pages for each member function so at least the up-to-date signature is browsable there.

I understand this may be quite time consuming, so maybe focus on just the content from the proposals for now. Examples can for sure be added later! Thank you!

Start with the existing cppreference std::mdspan page and compare it with P0009 and the accepted follow-up proposals. Add synopses for related classes such as layouts and accessors, then create browsable pages for their member functions with up-to-date signatures. Done means the relevant proposal content is represented; examples are explicitly optional.