I’m probably asking a stupid question, but I am in the process of writing an add-on that is not ready for release.
It will probably be at least a month, as I teach myself the ways of Webextensions.
I am trying to figure out how to set up a page before release, so that I can add notes, screenshots, etc. before it goes public.
I cannot find the addon page design tool anywhere.
I want do preview screen shots, validate a donate page, etc.
Hey there, @msaroff! This is not a stupid question at all. 
Currently, there is no way to set up a listing page prior to submission. However, it’s a great idea to start staging information you want to include in a listing before you submit! Before getting started, you might want to take a look at these tips for creating an appealing listing.
To set up donations for your extension, you just need to provide a URL that will point people to your donation page. (See this blog post for more info.) You’ll want to validate your page on whatever platform you want to use for collecting donations.
AMO will support screenshots with a maximum size of 1280x800 pixels, which is what we recommend you use. For other image sizes, we recommend a 1:6:1 ratio. You might want to curate a few screenshot options and test them out when you are ready to submit.
If you’re unhappy with the way things look when you submit your extension first set up the listing, it’s really easy to make updates.
OK, is there a page that shows how to actually design the page when I do my initial public release?
Not outside of the submission flow, no. Once you submit your extension to addons.mozilla.org, you’ll be asked to provide a summary, description, screenshots, and URL for donations.
I’m thinking that having something like Act Blue has for web pages might be a good thing.