Change 'Clusters' to 'Infrastructure' in navigation menu

At the request of the PMs, this MR renames the top-level menu item on the docs website from "Clusters" to "Infrastructure," and moves the "Infrastructure As Code" page (https://docs.gitlab.com/ee/user/infrastructure/index.html) into that section.

For @nagyv-gitlab - when we push this live, we need to communicate this change out to the company, especially to Support. I'll cc @cynthia (because she likes knowing about these things) and @lwbrown (because Support's stable counterpart to ~"devops::configure") to help get the word out.

Related to gitlab#223125 (closed)