Docs: link Docker Hub, the Unraid listing and SandObserver on the landing page

[SandObserver]

Summary

• Docker Compose and docker run install notes link the Docker Hub image sandobserver/posteryard.
• The Unraid install note links the Posteryard listing in Community Applications.
• The footer copyright and the "Also by SandObserver" heading link to backyard.sandobserver.com.

Testing

All green. Checked the page from 320 to 1440 px with no horizontal overflow.

Checklist

• Format, lint, type checks and tests pass.
• DESIGN_VERSION bumped and tests/golden written again if rendered images change.
• CHANGELOG.md updated under Unreleased for user-visible changes.

[cloudflare-workers-and-pages]

Deploying posteryard with    Cloudflare Pages

Latest commit:	6b3e627
Status:	✅  Deploy successful!
Preview URL:	https://ed7d3075.posteryard.pages.dev
Branch Preview URL:	https://docs-landing-listings-and-ma.posteryard.pages.dev

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