Docs: Update AI Catalog docs with the need for top-level group owners to enable agents/flows

• Start this issue's title with Docs: or Docs feedback:.

Problem to solve

For the Duo Agent Platform and AI Catalog documentation, we need to make sure the following is clear:

Enabling agents and flows requires two steps:

• A top-level group owner must enable them at the top-level group
• Project maintainers can then enable them for their specific projects

Another way to say this:

Project maintainers cannot enable individual agents and flows until a top-level group owner has enabled those individual agents and flows at the top-level group first.

Documentation locations that may need updating (approximate--there may be more)

To use agents and flows from the AI Catalog, you must have at least the Maintainer role for a project.

When you’re ready to interact with an agent, you can enable it to start using it with GitLab Duo Chat.

General Prerequisites section

• I feel as though here (or a similarly general area) we should really spell out that a manual process of asking the top-level group owner to enable an agent/flow is required.

Agents Prerequisites section

Enable an agent section

Enable a third-party agent section

I don't see any real outline of what to do in the Flows section.