Remove product from product-categories URL from template
What does this MR do?
Due to an upcoming URL change, this MR revises the use of https://about.gitlab.com/handbook/product/product-categories/-related links to instead be based on https://about.gitlab.com/handbook/product/categories/. The new link already works, so this is a non-breaking change.
This MR includes no other changes besides a URL change in the Documentation template.