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

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[luisherranz]

LGTM!

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Unlinked Accounts

The following contributors have not linked their GitHub and WordPress.org accounts: @Copilot.

Contributors, please read how to link your accounts to ensure your work is properly credited in WordPress releases.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Unlinked contributors: Copilot.

Co-authored-by: luisherranz <[email protected]>
Co-authored-by: juanmaguitar <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[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