DOC/STYLE: Fix "F821 undefined name 'pd'" errors
It'd be great to start linting the doctests (validate the docstring examples for pep8 issues).

To start with `core/generic.py`, `core/series.py` and `core/frame.py`, I created the next issues to address the current errors: #22892, #22893, #22894, #22895, #22896, #22897 and #22898.

After those are fixed, the only error that will remain in these files is `F821 undefined name 'pd'`. I checked for simple ways to avoid it, but I don't think we can prepend code for flake8 (like what we do in sphinx). So, the solutions I see are:

1. Add `import pandas as pd` (or even better `import pandas`) to every example
2. Do not lint the doctests
3. Ignore `F821` from the doctest linting (this will also ignore errors like `df + 1` where df is not defined, as it's also an `F821` error).
4. Implement a way to `grep -v` those exact errors, and change the exit code of `flake8` based on that, so the linting passes when the output is clean after filtering these errors

I know this has been discussed before, and I'm not sure if anyone else besides me is in favor of option 1. But I think it's worth considering it again in the context of linting doctests. Option 1 has the advantage that the examples are self-contained (they can be copy-pasted, or in the future we can even add a button to open them in binder, or something similar).