Add 26.4 Cloud release notes

[Blargian]

Stage the v26.4 Cloud changelog (curated from the 26.2→26.4 diff) and bump sidebar_position on existing release-note pages so 26.4 sorts to the top.

Summary

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/vercel.json
• 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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Note

Low Risk
Documentation-only changes to Cloud changelog pages and sidebar order; no product or runtime behavior.

Overview
Adds the v26.4 ClickHouse Cloud release notes at docs/cloud/reference/01_changelog/02_release_notes/26_4.md (/changelogs/26.4), with the usual Cloud-facing sections: backward-incompatible changes, new features, experimental items, performance, improvements, and a collapsible bug-fix list.

Navigation: sidebar_position is incremented by 1 on every existing Cloud release-note page (24.2–26.2) so 26.4 can use sidebar_position: 1 and appear first in the sidebar. The release-notes index table is regenerated to list v26.4 at the top.

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Project	Deployment	Actions	Updated (UTC)
clickhouse-docs	Ready	Preview	Jun 5, 2026 1:54pm