DOC: add documentation for read_spss(#27476)
- [x] closes #27476
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Documentation is added for the new read_spss function. 
After the generating html in Sphinx, it looks like this: 

IO page top:
<img width="766" alt="IO page" src="https://user-images.githubusercontent.com/6498024/61973737-c70bf580-afb2-11e9-95cf-499efc17e807.png">
SPSS description: 
<img width="774" alt="SPSS description" src="https://user-images.githubusercontent.com/6498024/61973739-ca9f7c80-afb2-11e9-8909-1638ad7a70d1.png">
API section:
<img width="761" alt="SPSS API" src="https://user-images.githubusercontent.com/6498024/61973743-cd01d680-afb2-11e9-9fca-be0a06128049.png">
