docs: add self-hosted HA guidance

[RainbowDashy]

Summary

• add a dedicated self-hosted HA guide
• update Kubernetes and production docs to describe the supported HA topology
• document the external PostgreSQL and license requirements, plus the current Helm chart limitation

Testing

• pnpm exec markdownlint docs/get-started/self-host/high-availability.mdx docs/get-started/self-host/deploy-with-kubernetes.mdx docs/get-started/self-host/production-setup.mdx docs/get-started/self-host/external-postgres.mdx docs/administration/license.mdx
• markdownlint still reports pre-existing MDX/component warnings in docs/get-started/self-host/external-postgres.mdx and custom component warnings such as /

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