Cloud: add documentation for secure connections to GCS

[Blargian]

Summary

Adds a guide for secure connection to GCS using gcs table engine and HMAC keys.
Addresses half of #5101.

Checklist

• Delete items not relevant to your PR
• URL changes should add a redirect to the old URL via https://github.com/ClickHouse/clickhouse-docs/blob/main/docusaurus.config.js
• If adding a new integration page, also add an entry to the integrations list here: https://github.com/ClickHouse/clickhouse-docs/blob/main/docs/integrations/index.mdx

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Project	Deployment	Review	Updated (UTC)
clickhouse-docs	Ready	Preview	Jan 7, 2026 11:19am

2 Skipped Deployments

Project	Deployment	Review	Updated (UTC)
clickhouse-docs-ru	Ignored	Preview	Jan 7, 2026 11:19am
clickhouse-docs-zh	Ignored	Preview	Jan 7, 2026 11:19am

[dhtclk]

Real small verbiage change, otherwise LGTM

[Blargian]

@morsapaes when you get a chance, would you mind giving a quick scroll through if it looks okay, if anything is missing? I'll try get around to doing the one for Azure later in the week.