CI/DOC: Add option to build whatsnew
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Adds the option to compile the whatsnew. It also builds the API (to resolve links) and opens the whatsnew page in a browser when complete. This is the most frequent reason why I build the docs, and cuts down build time by 40% on my machine.
@rhshadrach if you can merge master @jreback Merged and green. Thanks!