Move private let bindings to a better spot

[ryyppy]

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.