Provide better examples and remove outdating site edit references for the MainDashboardButton SlotFill

[ryanwelcher]

What?

The PR provides some better examples for the MainDashboardButton SlotFill and removes the outdated site editor example

Closes: #57870

Part of: #64749

[github-actions]

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Co-authored-by: ryanwelcher <welcher@git.wordpress.org>
Co-authored-by: justintadlock <greenshady@git.wordpress.org>
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[github-actions]

Flaky tests detected in f0ba41b.
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/10528923815
📝 Reported issues:

• [Flaky Test] Redirection after template creation #60678 in /test/e2e/specs/site-editor/site-editor-url-navigation.spec.js