dunder methods are not documented

[dhalbert]

adafruit_datetime defines a large number of "dunder" methods for date/time arithmetic and comparisons, but they are not listed in the documentation because they start with _. This is a general problem with Sphinx, though there are workarounds: see https://stackoverflow.com/questions/62142793/autosummary-generated-documentation-missing-all-dunder-methods-except-for-ini and sphinx-doc/sphinx#12858.

In the CPython documentation for datetime, written by humans, the effect of the dunder operations is listed in a big table: https://docs.python.org/3/library/datetime.html#datetime.timedelta.microseconds (following).

I told someone we didn't support date/time arithmetic, which was wrong, because I didn't see it in the documentation. We should figure out some way of making those operations visible, or list them by hand.