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 working in some cases - e.g. on Mac Chrome browser, 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 logging out of Google, or using a different browser in which you are not logged in to Google. Or you can edit the Google Doc directly]