docs(release-process): simplify and reorganize release instructions

- Remove dependency on the k8s release-notes CLI and related env vars
- Re-number and streamline setup: cmrel, gcloud, gsutil, cosign
- Consolidate release notes generation and add guidance to run make generate-release-notes
- Clarify GitHub release publishing, website PRs, and bump versions workflow

Signed-off-by: Richard Wall <richard.wall@cyberark.com>

Author: wallrj-cyberark