DOC: add value counts as related method to count
Counting values using `.value_counts()`, more generally, is a counting task, so it felt natural to link the `.count()` page to `.value_counts()`.

I copied the description for `.value_counts()` from the "See also" box in [`.drop_duplicates()`](https://github.com/pandas-dev/pandas/blob/9fed16cd4c302e47383480361260d63dc23cbefc/pandas/core/frame.py#L5102).

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