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
• Working with units
• Dual numbers
• Phase equilibria, for pure substances and for binary mixtures
  • different constructor methods
  • phase diagram with plots
• Critical point calculation for pure substances and mixtures
• DFT / PDGT
  • 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.