Suggestions for additions to user docs

[rhornung67]

• Use of camp::list to hold a set of numerical values in addition to types
• Use of camp::cartesian_product

[kab163]

I would add

• Find good user-facing examples of using Camp resources, event, EventProxy, tuple, etc. to add to the docs
• Add an example of using the data() method in Camp arrays in a similar way where you have to derefence the pointer (via [] or *)
• Add an example of how get can return const and non-const lvalue references, and rvalue references for Camp arrays
• Add an example that shows what a "listable construct" is on the Camp list feature page
• Double check: The Doxyfile file appears to be unused. blt_add_doxygen_target uses CMake's configure_file() to read Doxyfile.in, perform string substitution, and write Doxyfile in the build directory. (extern/blt/cmake/SetupDocs.cmake:30)

(These are some of the comments from PR #158 that were pushed to a future PR)

[trws]

• Use of camp::list to hold a set of numerical values in addition to types

This one is a bit funky, which is a great reason to document it. The camp::list type can't hold a list of numbers directly, camp::idx_seq<0,1,2> or camp::int_seq<short,0,1> can do that, but list has to wrap everything in a type to work. Converting one of those to a list with as_list<idx_seq<0,1>> makes list<integral_constant<idx_t, 0>, integral_constant<idx_t, 1>> or to use the alias list<num<0>, num<1>>. This is often useful though because then everything is a type, no issues with non-type template parameters, so those numbers can go through any of the algorithms that expect types and work.