Add enable Confluence integration Call To Action

What does this MR do?

This merge request adds a button to enable the Confluence integration added in !36262 (merged) on the Wiki /home page, when the Wiki is empty (has no pages yet).

Note: This MR will remain a WIP until its upstream !36262 (merged) is merged.

• Issue: #220934 (closed)

QA steps:

1. Enable the :confluence_integration feature flag
2. Have a project with a wiki, but no wiki pages created yet
3. Visit the project wiki (click Wiki on side nav)
4. New button should display, and when clicked, should link to the new Confluence integration
5. Click the button to go to the Confluence integration

Screenshots

Notice the additional text, and new button:

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