docs: publish Zensical documentation site

[lan496]

Summary

• add a Zensical documentation site branded with a symbol-only SpinForge logo, with generated API reference, local preview/build commands, and a least-privilege GitHub Pages workflow using dorny/paths-filter
• organize authored documentation around reader prerequisites and intent, put the first-read learning section first in the sidebar, separate authored technical references from the generated API reference, and keep duplicated guidance out of the README
• show the permitted paper schematics as direct PNG previews, retain the notebooks under docs/paper/, link to rendered notebook views, and configure MathJax for equations

Test plan

• uv lock --check
• just docs-build (strict Zensical build)
• crawl generated documentation and verify internal links and assets return successfully
• uv run prek run --all-files
• just test (174 passed)

Implements the locally verifiable work in #13. The issue remains open to confirm the live GitHub Pages deployment after merge.