The main page area is a Google Doc, embedded in this page. It is editable in box below (desktop only), or by editing this Google Doc.
[note 2018-11-06: editing within page is reported not work in some cases (mainly Mac Chrome, when you are logged in to Google). You may see error message, "An embedded page at docs.google.com says:" hit CANCEL. Or Google Doc may not load. If it keeps happening, try it when not logged in to Google, e.g. using a different browser. Or you can edit the Google Doc directly]