docs: add Docker Swarm HA guide for self-hosted Enterprise

[SunsetDrifter]

Summary

Adds a new guide at /selfhosted/enterprise/docker-swarm: Running NetBird HA on Docker Swarm. It is a companion to Running a Highly Available Self-Hosted Deployment and covers only what changes when that architecture runs on Docker Swarm instead of plain VMs:

• Why Swarm's routing mesh and default service networking (endpoint_mode: vip) don't fit the HA architecture, and what fills the gaps (Traefik for the application pools, HAProxy for PostgreSQL/Redis primary routing)
• Swarm-native PostgreSQL (Patroni/Spilo + etcd) and Redis (Sentinel) clusters, including the endpoint_mode: dnsrr requirement and other non-obvious pitfalls
• Full docker service create commands for all pools, Traefik labels for the gRPC/HTTP split, and an end-to-end failover verification checklist with measured expectations

Second commit is a separate grammar/readability editing pass over the new page.

Notes for review

• The page is intentionally not added to NavigationDocs.jsx — it should remain unlisted for now. It will still appear in sitemap.xml/llms.txt since those generators scan src/pages/ directly.
• New image asset: public/docs-static/img/selfhosted/enterprise/docker-swarm-ha-topology.png (uses the standard imagewrapper-big zoom class).
• All cross-page anchor links target the main HA guide and were checked against its actual heading slugs.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/docker-swarm-ha

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