docs: add GitHub CTA, About section, and SEO metadata

[Bowenislandsong]

Motivation

• Make the project repository and author information easy to find from the site so visitors can quickly access the open-source code.
• Provide clear ownership/context by adding a short About section that links to the personal profile for the project author.
• Improve search discoverability and structured data so search engines can better index project and author information.

Description

• Updated docs/index.html to add a prominent "Open Source on GitHub" button in the hero, an About nav anchor, an About us section linking to https://bowenislandsong.github.io/#/personal, and an author meta tag plus Person JSON-LD entry.
• Updated docs/_layouts/default.html to add a persistent GitHub link in the shared docs header navigation.
• Updated docs/site.css to include the new .about section in the global spacing rules and to add styles for .about and .about-copy for consistent layout and typography.
• Kept existing layout and assets intact while ensuring new markup uses rel="noopener" and target="_blank" for external links.

Testing

• Ran git diff -- docs/index.html docs/site.css docs/_layouts/default.html to review the changes, which returned the expected diff and succeeded.
• Verified working tree and staged changes with git status --short and added files, which showed the modified files and succeeded.
• Committed the changes via git commit -m "docs: add GitHub CTA, about section, and SEO metadata", which completed successfully.
• Inspected the updated files with nl -ba/sed to confirm the inserted nav item, hero CTA, About section, and JSON-LD entries are present and correct, which succeeded.

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Codex Task