Adding interactive text

You could use a button like this:

Button["Toggle cell below", SetOptions[NextCell[], CellOpen -> ! CurrentValue[NextCell[], CellOpen]]]

You can set the option "ShowGroupOpener" to True for the first cell of a cell group to add an Opener to open/close the cell groups. Alternatively, set the option "WholeCellGroupOpener" to True to make the whole cell a behave like a toggler button.

You can do this in two ways:

Manually: Right-click the cell bracket and select Properties to open the Option Inspector notebook. ClickCell Options >> Display Options and change the setting for "ShowGroupOpener".

Programatically: In the first cell of the notebook, using some identifying text for the cells to be modified ("comments" in the example below), execute

SetOptions[NotebookFind[EvaluationNotebook[], "comments", All], 
 "ShowGroupOpener" -> True]