DOC: fully document Index types

[jreback]

• add to API.rst
  • Int64Index
  • Float64Index
  • PeriodIndex
  • MultiIndex (DOC: Put MultiIndex into the main API Reference #11893)
  • RangeIndex (ENH: RangeIndex redux #11892)
• add to main docs (CategoricalIndex / Float64Index described in MultiIndex / Advanced Indexing sections). Expand this section to include.
  • Index
  • DatetimeIndex/TimedeltaIndex/PeriodIndex (already in timeseries.rst/timedelta.rst, need to cross-link to new section)
  • Int64Index
  • RangeIndex

cross-link from internals.rst about the Index internals.