Add DAG example for curve fit

This MR updates the documentation with an example for curve_fit and adds the doc infrastructure to add more examples.

Can this MR be accepted?

• Documentation extended or updated
  • Add doc/data_io/examples.rst
  • In there, add an example for curve_fit
  • Add crosslinks from other parts of the docs
• No code changes (except tested includes)

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Reviewed & approved by @JulianWeninger

Related issues

Closes #156 (closed)