Make internal implementations "protected"

There is a lot of stuff in the package that is not supposed to be used by end users. However, it bubbles up in many places: IDE autocompletions, HTML docs search results, etc. I propose to systematically rename "internal" and "implementation" modules and classes by prefixing them with an underscore, e.g. `lsdb._dask`, `lsdb._nested`, etc. I also to propose to hide "public" stuff which is reimported in the top `__init__.py`.

Also see #767 for API HTML docs improvements.

Begin by inventorying the package's internal and implementation modules and classes, including the examples `lsdb._dask` and `lsdb._nested`, along with public objects reimported in the top `__init__.py`. Review issue #767 for related API HTML documentation context. Done means the intended internal names are consistently prefixed and unwanted public reexports no longer appear in IDE completion or HTML documentation search results.

Written by the indexing model from the issue text.