docs: add AI Assistant user guide

[dskarzh]

Document the in-platform AI Assistant for managing Calculated Fields and Alarm Rules in natural language: where to find it, the ask-questions and describe-the-outcome interaction modes, contextual awareness, the approval workflow, prompting tips, and scope and limitations. Includes screenshots and updates the four product wrapper descriptions.

Description

Type of change

• New / updated documentation page (src/content/docs/**)
• Shared include file (src/content/_includes/**)
• Component, layout, or styling change (src/components/**, src/styles/**)
• Landing / use-case / case-study page (src/pages/**, src/data/**)
• Redirects (src/data/redirects.ts)
• Version bump / release (see release skill)
• Build, CI, scripts, or tooling
• Other (please describe)

Affected products

Related issues

• Closes #

Checklist

• pnpm check passes (Astro / TypeScript)
• pnpm lint:eslint passes
• pnpm lint:slugcheck passes (required if pages were added/renamed/moved across languages)
• pnpm lint:linkcheck passes locally — required to merge; run it before requesting review (use pnpm lint:linkcheck:nobuild if you already ran a build)
• Renamed/removed pages have a redirect in src/data/redirects.ts, and pnpm generate:redirects was run
• No hardcoded versions — values come from src/data/versions.ts
• Screenshots attached for visual changes