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)

Comienza con el flujo de trabajo de CD introducido por PR 1946 y las instrucciones de release en RELEASE.md; compáralos con la sección verify-release-signatures de la documentación de instalación. Revisa el script verify_release como posible punto de integración y, a continuación, determina qué enfoque de firma está aprobado. Se considera terminado cuando los artefactos de release incluyen instrucciones de firma y la firma está integrada en el proceso de release.