Update the docs site

The site is Zensical, the successor of Material for MkDocs by the same team, configured in docs/zensical.toml. docs/AGENTS.md describes the site as a whole: the two product tabs, the landing page, the derived files, the redirect stubs, and the deploy. This skill is the mechanics of one page.

To decide whether a change needs a page, and which one, use the docs-audit skill.

Where a page lives

docs/docs/<product>/<directory>/<slug>.md. The file's path is its URL: desktop/inbox/flows.md is /desktop/inbox/flows/, and a directory's index.md is its root (desktop/getting-started/index.md is /desktop/getting-started/).

The nav has two tabs beside Home (index.md, the landing page), one per product, each with sidebar groups: