Now we use an own, mobile-first, responsive sphinx theme based on Bootstrap 4.
It makes navigation on the very long "Python module instaloader" page a lot
easier. Also, it looks better.
This solves all the problems we had with sphinx-bootstrap-theme.
- Consistent background color in all types of codes
- Improve dd (list item) padding
- Link to Index in global TOC in navbar
- Briefly describe Instaloader in `index.html` title
- Remove page contents (navbartoc) in index, genindex and installation
- Link to new pypi.org rather than old pypi.python.org
