add first problem for c++ to doc

[fnrizzi]

currently in the doc we have: https://pressio.github.io/pressio-demoapps/endtoend_py.html
but don't have the same thing for C++: https://pressio.github.io/pressio-demoapps/endtoend_cpp.html

we should add this (obviously we could just say that we expect a build to be ready and point to page) and we could use as starting point the test: https://github.com/Pressio/pressio-demoapps/blob/develop/tests_cpp/eigen_1d_euler_sod_explicit/main.cc

and we can use for plotting an adapted version of the script in: https://github.com/Pressio/pressio-demoapps/blob/develop/tests_cpp/eigen_1d_euler_sod_explicit/plot.py