ENH: add peek function
- [x]  closes #18691
- [x] tests added / passed
- [ ] passes black pandas
- [ ] passes git diff upstream/master -u -- "*.py" | flake8 --diff
- [x] whatsnew entry

added peek, which is essentially a concatenation of head and tail functions. The reason I add, is because often for sorted data, you are interested in looking at both the head and tail simultaneously to see the min/max range.