docs(platform): document optional db-autoscaler component + DatabaseHorizontalAutoscaler CR

[scooby87]

What

Cozystack is adding a new optional platform component, db-autoscaler (Database Horizontal Autoscaler), in cozystack/cozystack#3381. It ships as an opt-in packages/system/ package enabled via bundles.enabledPackages: cozystack.db-autoscaler, and introduces a new tenant-facing CRD DatabaseHorizontalAutoscaler (group autoscaling.cozystack.io/v1alpha1) that automatically scales the read replicas of managed databases (Postgres/MariaDB/Redis/MongoDB) from VictoriaMetrics load.

Why a docs follow-up is needed

The website's managed-apps reference generator only tracks packages/apps and packages/extra, so a new packages/system platform component gets no page automatically. The relevant hand-maintained docs are not updated by CI either:

• content/en/docs/next/guides/platform-stack/_index.md — the platform component list should mention the optional db-autoscaler.
• content/en/docs/next/operations/configuration/variants.md (and the bundles / enabledPackages docs) — document how to enable it.
• A short usage page for the DatabaseHorizontalAutoscaler CR (spec fields, supported engines, guardrails, dry-run), mirroring the design proposal.

References

• Implementation PR: feat(db-autoscaler): implement Database Horizontal Autoscaler operator cozystack#3381
• Design proposal: https://github.com/cozystack/community/tree/main/design-proposals/database-horizontal-autoscaling
• Package README with supported-engines table and caveats: packages/system/db-autoscaler/README.md in the PR.

This is filed as the downstream follow-up tracked in the cozystack/cozystack#3381 "Downstream repositories" checklist. It can be scoped to content/en/docs/next/ and backported if needed.