CEXT-6400: Add Renovate config scoped to admin-ui-sdk/v2 samples

[obarcelonap]

Description

Adds .github/renovate.json, based on the Renovate config used in adobe/commerce-checkout-starter-kit, restricted via includePaths to only the admin-ui-sdk/v2/** samples so dependency updates don't touch v1 samples or any other sample directory in the repo.

Related Issue

N/A

Motivation and Context

The repo has no automated dependency updates for npm packages today (the existing .github/dependabot.yml only covers devcontainers). Keeping the newly added Admin UI SDK v2 samples current with @adobe/aio-commerce-lib-app and @adobe/aio-commerce-sdk releases needs automation, without expanding scope to the rest of the repo's samples.

How Has This Been Tested?

Validated the config with renovate-config-validator (npx -p renovate renovate-config-validator .github/renovate.json), which passed successfully.

Screenshots (if appropriate):

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.