Interactivity API: mention client-side-navigation scaffold variant in getting-started guide

[Copilot]

The "Getting started with the Interactivity Router" section only referenced the default @wordpress/create-block-interactive-template scaffold, omitting the client-side-navigation variant added in #76331 — leaving users without a purpose-built starting point for router work.

Changes

• Promotes the client-side-navigation variant as the recommended entry point for new router projects, describing what it demonstrates (router regions, prev/next navigation, loading indicator, state-persistence stopwatch)
• Retains the default variant as a secondary option for developers adding navigation manually

# Quickest start — fully wired client-side navigation out of the box
npx @wordpress/create-block@latest my-interactive-block \
  --template @wordpress/create-block-interactive-template \
  --variant client-side-navigation

# Default variant — add navigation yourself following the steps below
npx @wordpress/create-block@latest my-interactive-block \
  --template @wordpress/create-block-interactive-template

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[luisherranz]

LGTM!

[github-actions]

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Co-authored-by: luisherranz <luisherranz@git.wordpress.org>
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[github-actions]

Flaky tests detected in 1817248.
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/23154644135
📝 Reported issues:

• [Flaky Test] Should render and migrate legacy openSubmenusOnClick blocks #75559 in /test/e2e/specs/editor/blocks/navigation-frontend-interactivity.spec.js
• [Flaky Test] post title and content are editable and blocks can be inserted #76524 in /test/e2e/specs/editor/various/post-content-focus-mode.spec.js