Fix lychee link-check failures for root-relative paths

[graffhyrum]

Summary

• Fix root-relative path resolution: Add symlink approach (ln -sfn site 2026) so lychee can resolve /2026/ prefixed URLs when checking local HTML files. MkDocs generates these paths from site_url, but lychee has no way to map them without a root directory hint.
• Fix broken schedule anchor: Correct --jupyterlite to -jupyterlite in data/schedule.yaml — MkDocs strips & entirely rather than converting to -.
• Fix spaker typo: Correct field name to speaker in schedule data (pre-existing bug, silent due to .get() usage).
• Ignore false-positive links: Add g.page (Google Maps 302 redirects) and local-only post-mortem pages to .lycheeignore.

Test plan

• just clean && just check passes with 0 errors
• grep 'python-in-the-browser-building-interactive-documentation-with-mkdocs-jupyterlite' site/schedule/index.html confirms anchor fix
• Verify no 2026 symlink remains after just link-check completes

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