fix: correct VERCEL_TEAM_ID from required to optional Written against a stale local checkout of the CLI repo that predated the actual fix. On current main, src/lib/providers/vercel/credentials.ts explicitly marks teamId required: false with a comment citing issue #207 ("every account, including Hobby, is a team... omitting it lets Vercel resolve against the default team"), and VercelProvider.ts no longer throws when it's missing. The "required in practice" callout this page shipped with was already wrong the moment it was written.
docs: add Vercel Setup guide, cross-link from Cloudflare/Fastly Vercel had no dedicated setup guide even though it's the default provider — general instructions were folded into Getting-Started.md while Cloudflare and Fastly each got their own thorough guide (credentials, day-to-day commands, provider quirks, troubleshooting, security). Adds parity, verified against real CLI source (init.ts, VercelProvider.ts, providerHelper.ts, promptForCredentials.ts, setup.ts) rather than copied from the other guides' assumptions. Notably documents that VERCEL_TEAM_ID is required in practice today, despite setup.ts/README describing it as optional for personal accounts — the runtime throws if it's missing.