[Block Library - Site Tagline]: Add example in block settings

[ntsekouras]

What?

Just adds an example in Site Tagline's settings, so it can have a preview and be shown in Style book.

[github-actions]

Flaky tests detected in 4c410bc.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4244436190
📝 Reported issues:

• [Flaky Test] should display (capture the) text from the currently active link even if there is a rich text selection #36495 in specs/editor/various/links.test.js
• [Flaky Test] should select by dragging into nested block #44673 in specs/editor/various/multi-block-selection.test.js

[andrewserong]

Apologies @ntsekouras it looks like I merged my PR that does the same over in #48300 (I added a placeholder attribute so that empty Site taglines don't show "Write a site tagline", so had to put the change in the index.js instead of block.json so that the string could be translated).