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
