Improving pipeline subscriptions screen

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR makes several changes to the CI/CD Pipeline Subscriptions view in project settings.

1. Adds a Question O link to documentation on using pipeline subscriptions so that users can easily find out how to use this feature.
2. Breaks out the subscriptions table into upstream and downstream sections to make it clear which subscriptions are which.
3. Removes the delete subscription button from the entries in the downstream table, because subscriptions cannot be deleted this way. (Fixes #349252 (closed))
4. Changed the header for the "Owner" column to "Namespace" because it lists the namespaces to which subscription project exist under, not the people who own the projects.
5. Changed the project names to links to make it easier to learn more about the projects and change subscriptions for downstream projects.
6. Show an empty state message if no pipeline subscriptions exist. (Thanks, @anna_vovchenko!)

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

Current view:

Proposed view:

Proposed empty state view:

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In GDK, go to the gitlab directory.
2. Switch to this branch: git checkout calebcooper-improving-pipeline-subscriptions
3. Restart GitLab.
4. Go to the CI/CD settings in a project with subscriptions and expand the subscriptions section.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.