docs: rebrand with the shared ISCC Zensical theme

[titusz]

Summary

Rebrands iscc.codes with the ISCC identity (Edition 01, September 2026) through a new shared Zensical theme, zensical_iscc/, built package-ready so the other ISCC documentation sites can adopt it and it can later be published as a theme.

• Paper/near-black colour schemes, self-hosted Readex Pro (body at 300) and JetBrains Mono, brand signature and favicon, navy code blocks in dark mode, labelled admonition colours
• Coral calls to action with near-black text, blue links, coral current-page marker; content capped at 1385px as before
• Theme carries the shared widgets: "Copy page / View as Markdown / Edit on GitHub" split button, the iscc.ai chat widget (chat = true), and a Plausible analytics provider
• scripts/gen_markdown_pages.py publishes each page's Markdown as index.md beside its HTML plus llms-full.txt; the deploy workflow runs it after the build
• docs/overrides/ and the variable TTF fonts are removed; docs/stylesheets/custom.css keeps only site-specific components
• Brand assets are verbatim copies from the brand kit and excluded from the whitespace hooks

Verification

• prek run --all-files, check_site_paths.py (source and built site), zensical build --clean all pass
• Light and dark schemes checked in Chrome on the home, concept, specification and resources pages
• Phone layout is CSS-reviewed only