Stand up e2eonrails.com docs + landing site

[justin808]

Roadmap task S2, decided 2026-07-04 (ADR-0002): docs + landing page at https://e2eonrails.com (apex; domain purchased 2026-07-04). No docs. subdomain.

Information architecture (from docs/roadmap/2026-07-e2e-on-rails-naming-decision.md)

/                                   What is E2E on Rails? (hero from the naming doc)
/getting-started
/cypress                            Using E2E on Rails with Cypress
/playwright                         Using E2E on Rails with Playwright
/factory-bot
/fixtures
/scenarios
/app-commands
/migration/from-cypress-on-rails    content = #220's guide

Requirements

1. Source of truth stays in-repo (docs/); the site builds from it (pick the same generator family used by sibling ShakaCode docs sites — check shakapacker.com/reactonrails.com setup first and match; static + low-maintenance beats fancy).
2. Homepage hero: verbatim "Recommended docs homepage hero section" from the naming doc.
3. Publish llms.txt at https://e2eonrails.com/llms.txt (see Add AGENTS.md, AGENTS_USER_GUIDE.md, and llms.txt (agent-native docs) #222).
4. SEO: pages must target the search intents listed in the naming doc (e2e tests rails, playwright rails, cypress rails, factorybot cypress/playwright rails); runner-specific pages carry the runner keywords.
5. Link the domain from: README (hero), RubyGems metadata (homepage_uri/documentation_uri — next release), shakastack.com + shakacode.com (see roadmap S1), and the migration guide.
6. Renewal rule (record in the site repo README): keep the domain after year one only if it has real traffic/links; otherwise fold back under ShakaCode.

Sequencing

After #221 (docs split) provides the per-topic files; migration page after #220.

Acceptance

Site live on the apex domain with the IA above; Lighthouse ≥ 90 on the homepage; llms.txt served; README + gemspec point at it.