feat(docs): add zensical documentation site

[eric-tramel]

Summary

• Add a Zensical documentation configuration for the repo and replace the old hand-authored docs/adding-a-plugin.md content with focused authoring, workflow, release, and landing pages.
• Keep the generated plugin catalog as part of the docs site while preserving the existing ddp catalog freshness check.
• Add a GitHub Pages documentation workflow that builds docs on relevant PRs, uploads a preview artifact, and deploys the built site/ directory from pushes to main.
• Add make docs, include docs in make all, lock Zensical as a dev dependency, ignore generated site/, and update README/PR checklist links.

Why

The repo currently has Markdown docs but no documentation site or Pages deployment path. Zensical is a small fit here because the repo already uses Python and uv, and its build can validate internal links in strict mode before deployment.

Validation

• make sync
• make docs
• make check
• make all