FIX: Documentation for for 0.14.1 change log

Missing code reference in change log documentation
Raise exception on non-unique column index in to_hdf for fixed format.

Fixes #7761.
TST: unicode compat for test_astype_unicode (GH7758)
BUG/COMPAT: pickled dtindex with freq raises AttributeError in normalize related ops
API: Add PeriodIndex.resolution
BUG: reset_index with MultiIndex contains PeriodIndex raises ValueError
fix length of underlinings.
BUG: is_superperiod and is_subperiod cannot handle higher freq than S
DOC: added nanosecond frequencies to doc
Docs: Be more specific about inf/-inf no longer being treated as nulls.
Merge pull request #7804 from sinhrks/nanodoc

DOC: added nanosecond frequencies to doc
Merge pull request #7800 from lexual/indexing_docs_structure_fix

Correct docs structure in indexing docs.
Merge pull request #7789 from sinhrks/normalize_pickle

BUG/COMPAT: pickled dtindex with freq raises AttributeError in normalize...
Merge pull request #7805 from lexual/docs_inf_as_null_note

Docs: Be more specific about inf/-inf no longer being treated as nulls.
Merge pull request #7802 from sinhrks/reset_index_bug

BUG: reset_index with MultiIndex contains PeriodIndex raises ValueError
Docs: Panel.dropna now exists, update docs accordingly.
Merge pull request #7803 from sinhrks/superperiod

BUG: is_superperiod and is_subperiod cannot handle higher freq than S
CLN: Simplify LinePlot flow
TST: add mixed-type panels & frames to generate_legacy_pickles

Don't generate them for pre-0.14.1 as they cannot be unpickled.
BUG: fix unpickling of some pre-0.14.1 pickles with non-unique items

Series, frames and panels that contain only one block can be unpickled
under the assumption that block items correspond to manager items 1-to-1.