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)

PR 1946 で導入された CD ワークフローと RELEASE.md のリリース手順から始め、インストール ドキュメントの verify-release-signatures セクションと比較します。統合ポイントの候補として verify_release スクリプトを確認し、その後、どの署名方式が承認されているかを判断します。リリース成果物に署名手順が含まれ、署名がリリースプロセスに統合されていれば完了です。