Redraw tutorial schematics and point to the documentation

- Redraw the wall, fin, lid-driven cavity, cantilever and solidification
  schematics in the documentation style, with boundary keys, and
  re-export their PNGs.
- Fix the printVersion() call in the code listings of 12 tutorials
  (printVersion is a string).
- Use "constantTemperature" and ["zeroGradient"] in the homepage and
  tutorials.
- Use u instead of φ in the solidification tutorial, matching the docs.
- Describe the documentation and link it from the How to use and
  Tutorials sections.

Author: nikoscham