I currently edit manually the cell contents of a notebook (Ctrl+Shift+A or Cmd+Shift+A) to "recycle" cells containing the ShowGroupOpener option. Is there an easier way to create documents hiding contents under that type of icon?
Unless you want group openers for all groups -- which you probably don't, since that would put one at the very top level for the entire notebook -- then you can edit the notebook's style sheet, select the kind of cell (Section, Subsection, e.g.) for which you want the group opener, and then use the Option Inspector on that cell in the style sheet to include ShowGroupOpener.
You could do this either for a particular notebook by using the menu command Format > Edit StyleSheet or you could make a copy of a standard style sheet, modify that, and then select it as the style sheet for whatever notebooks you choose. (I'd advise against directly modifying any of the Wolfram-supplied style sheets.)
I can give a more detailed explanation if you like.