Added intro, simple example and struct definition for nmod_poly docs.
Added intro, simple example and struct definition for nmod_poly docs.

jpflori/flint2,wbhart/flint2,dsroche/flint2,fredrik-johansson/flint2,jpflori/flint2,dsroche/flint2,fredrik-johansson/flint2,wbhart/flint2,jpflori/flint2,dsroche/flint2,fredrik-johansson/flint2,jpflori/flint2,wbhart/flint2,dsroche/flint2