Add community-maintained starters discovery page

[MDF-1951]

This PR proposes an alternative approach to Issue #185.

Rather than linking users to an external page of third-party starters, it introduces a dedicated "Community-Maintained Starters" experience within the start.spring.io UI.

Motivation

PR #261 previously attempted to address this issue by adding a Help menu link to an external starter catalog. During review, concerns were raised that the relationship between the selected Spring Boot version and the linked starter information was not obvious.

This implementation keeps users within start.spring.io and provides additional context around community-maintained starters.

What Changed

• Added a new Community-Maintained Starters view accessible from the Help menu
• Added a curated starter catalog backed by static metadata
• Added search and tag filtering
• Added Maven and Gradle dependency snippets
• Displayed the currently selected Spring Boot version with guidance to verify compatibility through each project's documentation
• Added unit tests for catalog integrity and filtering behavior

Notes

The catalog is intentionally small and serves as a discovery mechanism for community-maintained starters that are not part of the curated Initializr dependency catalog.

Compatibility remains the responsibility of each starter project and users are directed to official project documentation for verification.

The initial catalog contains a limited set of well-established community-maintained starters and is not intended to be exhaustive. The metadata-driven approach is designed to scale incrementally, allowing additional community-maintained starters to be proposed and reviewed through the normal pull request process without requiring changes to the UI implementation.