DOC: collect positional deprecations, remove duplicate DateOffset methods
The DateOffset duplication is discussed in #41586.  docbuild is segfaulting locally (independent of this PR), so will have to see if this works on the CI.
@jorisvandenbossche this is spitting out a bunch of warnings like `/home/runner/work/pandas/pandas/doc/source/reference/api/pandas.tseries.offsets.YearBegin.rule_code.rst: WARNING: document isn't included in any toctree` ideas on how to handle these? if can revist after we merge #41586 