docs: improve implementation discoverability

[charantejguniganti]

Summary

Related to #110.

The TUF website already provides implementation and system information in the Getting Started documentation, but this information is not directly discoverable from the homepage.

I initially considered a structured catalog and reusable component, but after inspecting the current site, I found that the existing Getting Started page already serves as the source of truth, so this PR focuses on improving its discoverability without introducing duplicate data.

What changed

• Replaced the homepage "Contribute" feature with "Implementations".
• Added a direct link to /docs/getting-started/.
• Added concise copy explaining that the page contains TUF implementations and systems.
• Reused the existing documentation rather than duplicating implementation data.

Why

This provides a clear discovery path for users looking to use TUF while keeping the existing documentation structure and avoiding additional maintenance overhead.

Validation

• git diff --check passes.
• Local Hugo build could not be completed on Windows because the repository's Bash-based prepare process is not compatible with the local Windows environment.
• GitHub Actions will provide validation in the repository's intended Ubuntu environment.