You have COLLAPSIBLE HEADINGS in Google Docs now, as long as you do 2 things!
Collapsible headings are a great feature in Google Docs. There are 2 things that you need to do if you want to have collapsible headings - 1) You need to format your headings with a Heading Style. You can select a Heading Style on the toolbar right next to where you select a font. You have 3 Heading Styles to choose from so be sure to select one of them. Also, notice that I inserted small pictures of red stars in my heading names. This did not affect the headings as long as I selected a Heading Style for the actual text that I typed for the headings. 2) You need to be in Pageless Mode. This can be done by clicking on File and then Page Setup. Select Pageless at the top of the Page Setup window and click OK. Once you're in Pageless mode, when you hover to the left of each heading, you'll see a down arrow. Click on the down arrow and the text below the heading will collapse. Now there will be an arrow to the left of each heading. If you click on it, the heading will expand and you'll see the text below the heading.