API: add 'level' kwarg to 'Index.isin' method

PERF: optimize MultiIndex.isin when level != None

DOC: update doc bits on Index.isin and MultiIndex.isin

DOC: merge isin sections for container & index classes

DOC: make isin a separate section

It's no longer a subsection of boolean indexing.
Merge pull request #7892 from immerrr/add-level-kwarg-for-index-isin

API: add 'level' kwarg to 'Index.isin' method