docs: brand the documentation site to match pghardstorage.org

[cybertec-adrian]

Summary

Brand the MkDocs documentation site to match pghardstorage.org: the website's navy + cyan palette (light + dark), the wordmark logo/favicon and a home-page hero, typography tuning, a footer with Imprint / Privacy / Accessibility links and a #CCM.openWidget cookie trigger, a right-hand mobile nav drawer, plus an SEO-friendly de-duplicated <title> and Open Graph / Twitter Card meta. All assets are repo-local; no new build dependencies.

Type

• Bug fix
• New feature
• Refactor (no behaviour change)
• Documentation
• Test infrastructure
• Packaging / release

Tests

• make check passes locally (vet + race tests)
• New tests added for the changed behaviour, or there's a clear reason none exist
• Integration tests pass (make test-integration) where touched

Compatibility

• No on-disk manifest schema changes (or: schema bumped + 24-month back-read preserved)
• No CLI / API contract changes (or: documented + bumped)
• No new external dependencies (or: justified in the description)

Checklist

• Maintainer-attribution authoring (Author: Hans-Jürgen Schönig <hs@cybertec.at>)
• No AI-attribution lines anywhere
• CHANGELOG.md updated under the unreleased section
• Comments explain WHY (not WHAT) where the code isn't self-evident