Improve documentation

The documentation is outdated. It would be nice to improve the documentation, with the following points:
- [ ] Tutorials, for every class of solvers
- [ ] Update the main page `gsStructuralAnalysis.dox` with the latest operators
- [ ] Update per directory in `src/` the examples listed
- [ ] Add all classes to `gsStructuralAnalysis.h` to simplify includes (see how it's done in `gsKLShell`)