Add example notebooks to documentation

[g-bauer]

Feel free to add more. I'd favor more separate notebooks with focused scope - makes it easier for user to find what they look for.

• [ ] For each equation of state: simple setup / parameter handling
• [x] Working with units
• [x] Dual numbers
• [ ] Phase equilibria, for pure substances and for binary mixtures
  • [ ] different constructor methods
  • [ ] phase diagram with plots
• [x] Critical point calculation for pure substances and mixtures
• [ ] DFT / PDGT
  • [x] Setup, density profile for vle, surface tension
  • [ ] Setup pdgt, surface tension
  • [ ] external potential, radial distribution function
  • [ ] external potential, adsorption, adsorption isotherm
  • [ ] external potential, solvation

[prehner]

With the number of example notebooks that are already there, we should find a structure that makes the maintenance as simple as possible while making it easy to find the right example for a given problem.