docs(site): refactor landing SEO, layout, and assets

[Tam-Leal]

Summary

• Refactors the gitdock.dev landing page into shared CSS/JS assets with improved SEO and semantics.
• Updates copy (How it works, security panel, Octicons) to match the standalone zip download flow.
• Keeps the tentacle preview block and production alpha/parallax behavior unchanged.

What changed

• Landing: site/css/common.css, site/css/index.css, site/js/common.js, site/js/index-hero.js
• SEO: dashboard_v2.jpg for OG/Twitter/JSON-LD, manifest.json, sitemap.xml, robots.txt → sitemap.xml
• Content: unified security panel, aligned install steps, updated feature/pain icons (incl. tasklist for repetitive chores)
• Download head: static manifest + common.css link
• Removed: obsolete site/sitemap.txt

Test plan

• Open site/index.html via local HTTP server (not file://) and verify tentacles + dashboard preview
• Check Features, Security panel, Hub, How it works, and CTA sections
• Confirm site/download.html nav/styles still look correct
• Validate site/manifest.json and meta tags in page source