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)

Commencez par le workflow CD introduit par PR 1946 et les instructions de release dans RELEASE.md ; comparez-les avec la section verify-release-signatures de la documentation d’installation. Examinez le script verify_release comme point d’intégration possible, puis déterminez quelle approche de signature est approuvée. Le travail est considéré comme terminé lorsque les artefacts de release disposent d’instructions de signature et que la signature est intégrée au processus de release.