Update and restructure release engineering documentation

[saschagrunert]

What type of PR is this:

/kind cleanup
/kind documentation

What this PR does / why we need it:

Two-part update to the release engineering documentation:

Content fixes (commit 1):

• Replace all SendGrid references with Gmail API (krel announce send)
• Fix invalid --type=release in krel examples (valid: alpha, beta, rc, official)
• Add missing --nomock flag to release cut steps
• Fix testgridshot description to match actual behavior
• Fill empty sections in go.md, update kube-cross docs
• Remove stale Content Notice warnings, broken links, TODO comments
• Update conventions to 1.36 release cycle
• Fix typos across multiple files

Structural restructure (commit 2):

• Merge branch-manager.md and patch-release-team.md into a single release-manager.md (roles were consolidated years ago)
• Extract security/CVE handling into handbooks/security-releases.md
• Reorganize file layout into handbooks/ (procedural) and reference/ (informational)
• Rename files for consistency: k8s-release-cut.md -> release-cuts.md, post-release-branch-creation.md -> branch-creation.md, go.md -> go-updates.md
• Replace Lucidchart SVG with mermaid diagrams
• Rewrite README.md as a proper documentation index
• Simplify cherry-pick approval docs to reflect current Prow plugin workflow
• Remove obsolete spreadsheet tracking section, redirect stub, unused PNG
• Update all cross-references across the repo
• Delete role-handbooks/ directory, platforms/guide.md, img/ directory

Which issue(s) this PR fixes:

Fixes #1324
Fixes #1649
Fixes #2291
Fixes #2796
Fixes #2797

Special notes for your reviewer:

New file layout:

release-engineering/
├── README.md                  (documentation index)
├── handbooks/
│   ├── release-manager.md     (merged from branch-manager + patch-release-team)
│   ├── release-cuts.md        (was k8s-release-cut.md)
│   ├── security-releases.md   (new, extracted from branch-manager)
│   ├── branch-creation.md     (was post-release-branch-creation.md)
│   └── go-updates.md          (was go.md)
└── reference/
    ├── artifacts.md
    ├── versioning.md
    ├── platforms.md            (was platforms/README.md)
    ├── baseimage-exception-list.md
    └── gcp.md

/sig release
/area release-eng

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: saschagrunert

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[saschagrunert]

@kubernetes/release-engineering PTAL

[saschagrunert]

@kubernetes/release-engineering PTAL