docs: restore Sealos community installation guide

[yangchuansheng]

Dependency: The live-validated template update is tracked in labring-actions/templates#731. This pull request remains a draft until that change is merged and the published template is verified.

Context

The original Sealos guide was merged in #1666 and removed in #2292 because the
available template was still pinned to an old 0.23 beta release.

The community template has now been rebuilt and live-validated with Headscale
0.29.2 and Headplane 0.7.0. This PR restores a version-neutral community guide
with an explicit maintenance and support boundary.

Changes

• add docs/setup/install/sealos.md
• add one installation navigation entry in mkdocs.yml
• document API-key login, node registration, persistence, SQLite, optional
  PostgreSQL, and upgrade handling
• direct deployment-specific issues to the Sealos templates repository

Validation

• Sealos template: https://sealos.io/products/app-store/headscale
• Template source: https://github.com/labring-actions/templates/tree/kb-0.9/template/headscale
• Tested deployment: 2026-07-24
• Headscale: 0.29.2
• Headplane: 0.7.0
• Verified: Headplane API-key login, user and pre-auth key creation, node
  registration flow, SQLite persistence, PostgreSQL persistence, and config
  recovery
• Persistence: /var/lib/headscale, /etc/headscale, and /var/lib/headplane
• Documentation build: mkdocs build --strict

Maintenance

I can help maintain this community deployment path and follow up when supported
Headscale versions, Headplane integration, or Sealos template behavior change.

• have read the CONTRIBUTING.md file
• raised a GitHub issue or discussed it beforehand
• added unit tests
• added integration tests
• updated documentation
• updated CHANGELOG.md