Update section titles in detection notebooks

[giovannivolpe]

Update the section titles and relative ToC in the detection notebooks:

• detection_spheres.ipynb
• detection_ellipses.ipynb
• detection_core-shell.ipynb

For example, from "Import the Required Libraries and Load Utility Functions" to "Importing the Required Libraries and Load Utility Functions".

This is for consistency with the Linking notebook.

Also., double-check that all links from the ToC work.