DOC: Make pyplot import explicit in the 10 minutes to pandas page
Beginners don't know what is `plt`, thus it is better to show how to import it explicitly.

This is a continuation of https://github.com/pandas-dev/pandas/pull/30274

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@datapythonista Could you please merge this PR? Thank you.
 > AttributeError: module 'IPython.utils.py3compat' has no attribute 'cast_bytes_py2'

pandas-dev.pandas (Web_and_Docs) is failing, but is it related to my change?

https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=24127&view=logs&j=5291ec4e-9543-503b-43ce-895cf0e06ba3&t=ba0a2b35-e1f8-54df-fd66-2b9fde6f4b1f&l=5158 >pandas-dev.pandas (Web_and_Docs) is failing, but is it related to my change?

I also have the same problem. See #30282 @yuseitahara can you merge master into your branch please? It should fix the CI. @jreback you're changes have been addressed, all green here. thanks @yuseitahara 