Rework GCP deployment doc for VM install, align with AWS/Azure (#470)

## Summary

Compute Engine no longer supports deploying a container as the VM OS
(the **Deploy Container** option at VM creation), so the [GCP deployment
guide](https://questdb.com/docs/deployment/gcp/#launching-questdb-on-google-compute-engine)
is reworked to:

- Create a standard Ubuntu 24.04 VM and install QuestDB over SSH using
the release binary, mirroring the Azure guide's install flow.
- Adopt the AWS/Azure page structure: **Quick reference** →
**Infrastructure** (instance sizing, storage, networking) →
**Deployment** → **Security** → **Operations** → **Enterprise on GCP**.
- Fold the GCP-specific storage content (Hyperdisk Balanced, Filestore,
GCS, NetApp Volumes) into the new sections, with Filestore/NetApp/GCS
now grouped under Enterprise replication.
- Remove the stale Pulumi example (Ubuntu 20.04, raw `docker run`,
hardcoded credentials; AWS/Azure have no IaC section).
- Add a note explaining that container-as-VM-OS is no longer supported.
- Delete the orphaned `create-vm-docker.webp` (deprecated flow) and
`instance-available.webp` screenshots.

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: sklarsa