DOC/TST: index followup
TST: argmin/max impl, tests, and doc-strings
DOC: release notes corrections
API: raise on `__nonzero__` for Indexes, closes  #7897

partial close of #7904 

@jorisvandenbossche this fixes the missing properties from api docs and such, however

some properties, that are `cached_read_only` are not having the doc-strings picked up

e.g. `Index.is_monotonic,is_unique`....

do you know what the problem is here?
 Nope, I don't really know what the exact cause is, it's a long standing issue, see #6100 (it happens in two cases: when an attribute is initialized as class attribute to None (eg Index.name), or when cache_readonly is used for properties (eg Index.is_unique))
 @jorisvandenbossche hmm, ok then
