[codex] Switch docs publishing to Zensical

[acecchini]

Summary

• pin docs setup-uv to the requested commit for v0.8.1
• make Zensical 0.0.43 the docs builder and remove direct MkDocs config/dependencies
• align Zensical config with the previous site nav, assets, theme variant, icons, and search proportions while keeping the Zensical footer credit
• refresh the lockfile to latest compatible packages

Validation

• uv --cache-dir .uv-cache lock --check
• uv --cache-dir .uv-cache run zensical build --clean
• uv --cache-dir .uv-cache run prek run check-yaml --files .github/workflows/docs.yml
• git diff --check
• uv --cache-dir .uv-cache run prek run -a
• uv --cache-dir .uv-cache run tox run -e dev
• visual comparison against the saved MkDocs reference homepage; layout/theme/search proportions match, footer credit intentionally says Zensical