Public categories are a great way to have pages that are accessible outside of your organization.
All pages in a Public Category will be public to the world wide web.
Creating a Public Category
Click on Categories in the lefthand sidebar, then click Create a category
Now give your category a name, and change the Who should be able to view this category? to The Public.
A little message will show warning you that all pages within this category will become public.
Creating a Public Page
All you need to do is create a new page, within your new public category, and that page will now be public to anyone outside of your organization.
Making a Private Category Public
You can change an existing category into a public one by clicking the kebab menu next to a category and selecting "Edit":
Once you are on this page, change the Who should be able to view this category? to The Public
Note: There is no way to have a Public Page in a Private/Team Category. All pages in a public category will be public, and all pages in a private category will be private.