Rename GCP to Google Cloud and standardize all 6 VPS docs to follow
a consistent structure: Goal, Prerequisites, Quick Path, Step-by-step,
Persistence, Cost, Troubleshooting, See Also.
Changes:
- Rename gcp.md to google-cloud.md with redirects
- Add all 6 VPS platforms to navigation (alphabetical order)
- Standardize section headings across all docs
- Add missing sections (Cost, Troubleshooting, See Also) where needed
- Keep existing detail levels and friendly variants preserved
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