cd: re-enable release signing

Description of issue or feature request:
https://github.com/theupdateframework/python-tuf/pull/1946 adds a CD workflow to release build artifacts on PyPI and GH upon successful completion of the CI workflow for a pushed release tag.

The PR also removes instructions from RELEASE.md to gpg sign release artifacts and add them to the GitHub release assets as part of the previously manual release process. However, the installation docs still mention release signatures.

Current behavior:
No instructions / release process integration to sign release artifacts

Expected behavior:
Add instructions to sign release artifacts and integrate with release process

Ideas:

• quick-fix 1: sign in GitHub CD action
• quick-fix 2: sign locally and upload signatures to release assets manually (bonus: integrate signing/uploading with verify_release script)
• long-term fix: e.g. in-toto (https://github.com/theupdateframework/python-tuf/issues/529)

Inizia con il workflow CD introdotto da PR 1946 e le istruzioni per la release in RELEASE.md; confrontali con la sezione verify-release-signatures della documentazione di installazione. Esamina lo script verify_release come possibile punto di integrazione, quindi determina quale approccio alla firma è approvato. Il lavoro è completato quando gli artefatti della release includono istruzioni per la firma e la firma è integrata nel processo di release.