Add "Contribute translations" to CONTRIBUTING.md

[NiklasBuchfink]

This PR adds a "Contribute translations" section to the CONTRIBUTING.md.

Content:

• link to inlang localization editor
• link to "how to use" guide
• badge with current translation status

Preview badge:

This will help contributors discover and use the Fink localization editor to add missing translations or fix typos.

[saadeghi]

Thank you!