BUG: Correct importing behavior for Categoricals in StataReader

Ensure that category codes have the same order as the underlying Stata data.
Also adds a flag that allows categorical data to be treated as ordered or
unordered when importing.
BUG: fix negative step support for label-based slices

INT: make Index.slice_locs step-aware

BUG: fix PeriodIndex.searchsorted to accept Periods

INT: refactor time-related indices to use step-aware slice_locs

INT: refactor MultiIndex to use step-aware slice_locs

INT: enable second/microsecond partial string slicing
Merge pull request #8836 from bashtage/stata-monotonic-categoricals

FIX: Correct Categorical behavior in StataReader
Clarify encoding kwarg on to_csv
DOC: Clean up Stata documentation

Clean up and simplification of Stata documentation.

[skip ci]
Merge pull request #8753 from immerrr/refactor-slice-locs

API: allow negative steps for label-based indexing
Merge pull request #8858 from bashtage/stata-doc-fixup

DOC: Clean up Stata documentation
Merge pull request #8857 from graingert/patch-1

Clarify encoding kwarg on to_csv
BUG: type change breaks BlockManager integrity