[doc] Project modularisation in conjunction with Dune

[cuihtlauac]

This is a tutorial explaining how to combine some of the mechanisms provided by Dune with those existing in the language, to organize projects into modules.
Documentation already exists on the OCaml module system. However, there is little or nothing on how OCaml sources files can be written, organized in folders, named and referred to by dune configuration files to expose a structured set of module interfaces.

This includes:

• Making a module private with or without writing a .mli file
• Flattening a module hierarchy to expose definitions at the same level
• Extending existing modules
• Migrate dependencies using functors
• Vendoring components
• Shortening names
• Articulating names: module, file, folder and definition