DOC: read_excel doc - fixed formatting and added examples
Fixes a formatting bug in the `read_excel` docs that caused a line break and bold print in list of `_NA_VALUES`. Adds examples in the `read_excel` docstring.

- [x] closes #18735 
- [x] tests added & passed
- [x] passes `git diff master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
