DISC/REF: split frame.py and generic.py into smaller files

[jbrockmendel]

These files are big enough that GH has trouble displaying a blame. I think we should consider splitting pieces of these off into other files. Brainstorming:

• _make_logical_function, _make_cum_function, _make_stat_function_ddof, _make_stat_function, _make_min_count_stat_function at the bottom of generic could have their own file, maybe ops.unary
• shared_docs could go in a dedicated file (this might also avoid some circular import issues IIRC)
• IO pass-through methods could go into mixin classes (xref [ENH] Add to_markdown method #30350 motivating this idea)
  • to_json, to_hdf, to_sql, to_pickle, to_clipboard, to_xarray, to_latex, to_csv make up ~1200 lines of generic
  • to_gbq, to_stata, to_feather, to_parquet, to_html make up ~450 lines of frame.py

Thoughts?

[jreback]

sure this sounds reasonable

shared docs and/or io would be a good place to start

[jreback]

we should still do this :)

o would split out the io stuff first as that has huge doc strings