docs: add Fastly Next-Gen WAF coverage New Fastly-Setup and Fastly-Migration pages mirroring the existing Cloudflare ones, registered in the sidebar. Updated Home, Getting-Started, Configuration, Commands-Overview, and CI-CD-Integration to mention Fastly alongside Vercel/Cloudflare wherever they previously only covered two providers — condition type/operator/action compatibility tables, env vars, CLI examples, and the multi-provider CI pipeline example. Companion to gfargo/doorman#204/#205 (Fastly provider adapter).