[c++/en] Needs a chapter explaining difference between header and source files (compilation unit).

How to properly divide code between header and source files is really common problem when working with C or C++ for the first time. Tutorial should explain basic idea of this declaration and definition separation and provide easy general rules for working with multiple compilation units.

Start by locating the C++ tutorial or chapter structure in this repository and review how new language documentation is organized. Add a chapter explaining declaration versus definition, header and source files, compilation units, and practical rules for splitting code across multiple units. Done means a beginner can understand the separation and apply the guidance.

索引モデルが issue の本文から書いたものです。