Add help articles: Docker quick-start and SEO/sharing

#42 in Riparion/riparion-cms — merged 2026-06-02

Summary

Adds two build-embedded articles to the public help system (src/pages/reader/help/):

• Getting started with Docker (Getting started section, /help/docker) — a reader-facing quick start for the shipped Docker image: docker compose up --build, seeding demo content with DX_SEED, where data persists (./data, ./uploads), and going live with SITE_URL. Complements the developer-oriented docs/docker.md.
• Search engines & social sharing (new Reach section, /help/seo) — documents the SEO features the app provides automatically (SSR title/description, Open Graph + Twitter Card tags, featured-image previews, article metadata, /sitemap.xml, /feed.xml + autodiscovery and redirect aliases) and the knobs authors control (SITE_URL, site title/tagline, excerpt, featured image, slug).

Both are registered in HELP_ARTICLES, so on a fresh seeded instance they also become demo posts with categories/tags derived from their front matter (existing seed wiring; no code changes needed there).

Verification

• cargo fmt --check ✅
• cargo check --features server ✅
• cargo clippy --no-default-features --features server,sqlite --all-targets -- -D warnings -A clippy::too_many_arguments ✅

Content-only change (markdown + a registry append); no postgres/wasm-specific code touched.

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