Move private let bindings to a better spot

There seems to be a section about %%private let bindings in the let binding section, but it kinda mixes up a few concepts that weren't covered yet (public / private, modules, data hiding):

https://rescript-lang.org/docs/manual/latest/let-binding#private-let-bindings

This section should be kept way shorter, and then forwarded to an advanced section explaining data hiding in more detail, when all the major concepts have been introduced from a chronological standpoint.

Start by reading the private let bindings section at the linked let-binding documentation page and inspect the surrounding manual structure for the appropriate advanced section. Shorten the current section, move the detailed discussion of public/private visibility, modules, and data hiding to a later section, and verify that the documentation follows a chronological introduction of these concepts.