Explain `bin` and `lib` folder naming convention

[cuihtlauac]

In Unix folders named:

• bin contains compiled executable files (e.g. /bin or /usr/bin)
• lib contains compiled library files (e.g. /lib or /usr/lib)

In OCaml/Dune projects those folders often contain sources corresponding to executables or libraries.

Tutorials on executables, libraries, modules and Dune should clarify this.

From: https://discuss.ocaml.org/t/sorry-not-getting-it-with-mli/14883

Also, tutorials use Dune without explaining the role of the _build folder, which is problematic.