DOC: add option for skipping execution of code blocks
Came up as an idea at the documentation meeting (https://github.com/pandas-dev/pandas/issues/39183). This should make documentation builds faster for people who aren't working on the code samples.

```
$ time python make.py --single getting_started/comparison/comparison_with_spreadsheets.rst
…
real    0m22.119s
user    0m13.745s
sys     0m1.700s
$ time python make.py --no-ipython --single getting_started/comparison/comparison_with_spreadsheets.rst
…
real    0m17.481s
user    0m11.750s
sys     0m1.521s
```

Very open to alternatives names to `--no-ipython`.

- [ ] ~~closes #xxxx~~
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] ~~whatsnew entry~~
