Refactor and restructure environments page

- Groups content into relevant sections.
- Content edited.

doc/ci/environments/protected_environments.md

+# Protected Environments **[PREMIUM]**
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6303) in [GitLab Premium](https://about.gitlab.com/pricing/) 11.3.
+
+## Overview
+
+[Environments](../environments.md) can be used for different reasons:
+
+- Some of them are just for testing.
+- Others are for production.
+
+Because deploy jobs can be raised by different users with different roles, it is important that
+specific environments are "protected" to avoid unauthorized people affecting them.
+
+By default, a protected environment does one thing: it ensures that only people
+with the right privileges can deploy to it, thus keeping it safe.
+
+NOTE: **Note**:
+A GitLab admin is always allowed to use environments, even if they are protected.
+
+To protect, update, or unprotect an environment, you need to have at least
+[Maintainer permissions](../../user/permissions.md).
+
+## Protecting environments
+
+To protect an environment:
+
+1. Navigate to your project's **Settings > CI/CD**.
+1. Expand the **Protected Environments** section.
+1. From the **Environment** dropdown menu, select the environment you want to protect.
+1. In the **Allowed to Deploy** dropdown menu, select the role, users, or  groups you want to have deploy access.
+   There are some considerations to have in mind:
+    - There are two roles to choose from:
+      - **Maintainers**: will allow access to all maintainers in the project.
+      - **Developers**: will allow access to all maintainers and all developers in the project.
+    - You can only select groups that are associated with the project.
+    - Only users that have at least Developer permission level will appear on
+      the **Allowed to Deploy** dropdown menu.
+1. Click the **Protect** button.
+
+The protected environment will now appear in the list of protected environments.
+
+## Modifying and unprotecting environments
+
+Maintainers can:
+
+- Update existing protected environments at any time by changing the access on **Allowed to deploy** dropdown menu.
+- Unprotect a protected environment by clicking the **Unprotect** button of the environment to unprotect.