Add example to overlay_segments help text

[ammar257ammar]

This is related to Part 2 "Overlay segments" of pitch https://github.com/DIAGNijmegen/rse-roadmap/issues/362.

The overlay_segments is used in three places:

• It is used in OverlaySegmentsMixin which is used in:
  • ComponentInterface model
  • Question model (reader studies)
• It is used in the WorkstationConfig model.

This PR adds a default JSON example and a link to documentation to the overlay_segments field.

[ammar257ammar]

Since the example added to the overlay_segments form field is fixed (hardcoded), is a form test still recommended to check it is there in the help_text?

[amickan]

Since the example added to the overlay_segments form field is fixed (hardcoded), is a form test still recommended to check it is there in the help_text?

No, we don't normally test static helptext.