Docs: Ensure all admin steps have prerequisite

Problem

There are some tasks in GitLab that can only be carried out by an instance administrator. Typically, task instructions start with:

1. In the upper-right corner, select **Admin**.

This button is not visible to non-administrators. However, users search the documentation for instructions, and land on a set of steps that doesn't work for them, and they assume the docs are incorrect.

Example: https://docs.gitlab.com/integration/oauth_provider/#create-an-instance-wide-application

Proposal

Add a clear prerequisite to every task that accesses the Admin area, using the guidelines in the style guide.

For example:

Prerequisites:

- You must be an instance administrator.

Considerations

We assume that any task that is under the Administer section of the top nav in the docs is for administrators only. Therefore do we need to include the prerequisite for every one of those tasks in that section of the nav?

Users could still land on one of those pages through search, and not realise they are in the admin docs, so there might be some value to calling it out.

Do an audit of these pages first to determine best course of action.