API: offsets.Day is always calendar-day
- [x] closes #44823
- [x] closes #55502
- [x] closes #41943 
- [x] closes #51716
- [x] closes #35388
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Alternative to #55502 discussed at last week's dev meeting.  This allows TimedeltaIndex.freq to be a`Day` even though it is not a Tick.


`/home/runner/work/pandas/pandas/doc/source/whatsnew/v3.0.0.rst:345: WARNING: Bullet list ends without a blank line; unexpected unindent. [docutils]`

is stumping me.  Who is our go-to person for debugging these?
 Still seeing 

```
/home/runner/work/pandas/pandas/doc/source/whatsnew/v3.0.0.rst:345: WARNING: Bullet list ends without a blank line; unexpected unindent. [docutils]
```

Tried building the docs locally and jinja2 is complaining `ValueError: PackageLoader could not find a 'io/formats/templates' directory in the 'pandas' package.` so doing some yak-shaving. Looks like there was even more whitespace I messed up.  It's happy now. Thanks @jbrockmendel. Good to finally have this change!  Looks like we missed a usage of freq.nanos in the window code.  I don't know that code too well.  Does that need updating too? Hmm I think we always treated `"D"` as 24 hours before this change, and you defined `Day.nanos` in this PR so that's probably why the tests were still passing (and that we might not having any rolling tests with DST?).

I guess technically with this change `rolling("D")` shouldn't work since `Day` isn't a fixed frequency anymore, but maybe we should keep allowing this case?

 > and that we might not having any rolling tests with DST?

looks like we have one rolling test (test_rolling_datetime) with Day and tzaware self._on but i don't think it passes over a DST transition