DOC/CLN: Index shared docs with appending actual docstrings
xref https://github.com/pandas-dev/pandas/issues/19932 and https://github.com/pandas-dev/pandas/pull/20016

This removed the `_index_shared_docs` dict for the Index class. 
There were 4 methods that actually substituted values: take, get_indexer, get_indexer_non_unique, fillna