Add empty state to Deploy Keys admin section

What does this MR do?

This MR:

• adds an empty state with SVG icon for the Admin Deploy Keys section.
• styles the New deploy key button as a successful one to make it coherent with how the button is displayed in other sections (see screenshots).
• center-aligns the layout – instead of the right-aligned proposal – to make it coherent with GitLab UI Empty State Regions.

Related issue: #36349 (closed)

Screenshots

Empty	Populated

Labels	Deploy keys (new)

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team