Unfortunately, at the moment there is no way to arrange the order of the Prismic document list alphabetically (or any other way).

How do we arrange document list in Prismic

We use last update time or date to arrange the document list. It means you will get the most recently published documents at the top and the oldest at the end of the list.

Workaround

We recommend setting up the in-website edit-button and using that to more quickly find your pages in Prismic. This will allow you to create a button on each page of your website that will link directly to that page in in your Prismic repository. 

This makes it easy to find the page you want to edit on your own website then navigate directly to that page in Prismic.

Here is the link to setup the in-website-edit-button on your website. This following is a link to the Javascript documentation, but this is available in other languages as well:
https://prismic.io/docs/javascript/beyond-the-api/in-website-edit-button

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