CLN: Change assert_(a [not] in b) to specialized forms

Work on #6175. Changes instances of assert_(a [not] in b) to specialized assert[Not]In(a, b).
ENH: Add functions for creating dataframes with NaNs
DOC: link to pandas-cookbook instructions

Some people were confused about how to use this cookbook, so adding a link to the setup instructions.

see: https://github.com/jvns/pandas-cookbook/issues/7
this SO question: https://stackoverflow.com/questions/21868105/problems-with-pandas-tutorial-missing-file-bikes-csv-and-special-characters-in
Merge pull request #6435 from jvns/patch-2

DOC: link to pandas-cookbook instructions
PERF: Perf issue in concatting with empty objects (GH3259)
API: concat will now concatenate mixed Series and DataFrames using the Series name
     or numbering columns as needed (GH2385)
Merge pull request #6438 from jreback/concat

PERF/API: concat improvements
DOC: add explanation to doc/sphinxext
DOC: small doc fixes
Merge pull request #6442 from jorisvandenbossche/doc-fixes

DOC: small doc fixes
Merge pull request #6395 from bwignall/assert_6175_innotin

CLN: Change assert_(a [not] in b) to specialized forms
TST: fix spurious tests for test_groupby (GH6436)
BUG/TST: iloc will now raise IndexError on out-of-bounds list indexers to promotoe
         consistency with python/numpy syntax. The out-of-bounds for slice indexers
         will continue to work (again for consistency) (GH6296 / GH6299)
DOC: fix .tz attribute error in DatetimeIndex when building docs
Merge pull request #6443 from jreback/iloc_bounds2

BUG/TST: iloc will now raise IndexError on out-of-bounds list indexers (GH6296 / GH6299)
CLN: minimize tokenizer passes
Merge pull request #6432 from cpcloud/eval-consolidate-tokenization

CLN: minimize tokenizer passes
INT/CLN: clean up block slicing semantics
Merge pull request #6446 from jreback/iloc_tests

INT/CLN: clean up block slicing semantics
str_extract should work for timeseries, bug 6348