WEB: Added theme dependent favicon 
- [x] closes #42314
- [x] 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
- [x] whatsnew entry

Added a light mode version of the favicon based upon the .svgs in the static folder as well as added a small js scipt to the template page to detect the theme change.

![mgCmqcreiy](https://user-images.githubusercontent.com/53790060/123965934-fe1c8a00-d9c5-11eb-8838-ce72ee988a3c.gif)
