Resolve "Add no graph empty state for DAG"

What does this MR do?

Adds a new empty state for the DAG graph view in the CI space that only get showns if there are no needs at all. If there is only one, we render a warning message instead (this is already implemented).

We add a SVG and some text that explains how to get started with needs jobs. We also link to the documentation with the big green button at the end. You can refer to #217648 (closed) for design reference.

Screenshots

Before:

After:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing