chore: remove Cloudflare Access section from public docs

[suisuss]

Summary

• Removes the "Gated Environments (Cloudflare Access)" section from `docs/concepts.md`.
• Documentation-only change: no Go code, tests, or behavior touched.
• The CF Access feature continues to work — `CF_ACCESS_CLIENT_ID`/`CF_ACCESS_CLIENT_SECRET`, `CF_AUTHORIZATION`, and per-host `hosts.yml` `headers:` maps still flow through `internal/http/cfaccess.go` and the shared HTTP client.
• Rationale: the public-facing CLI guide should not advertise the Cloudflare gating mechanism that fronts our environments.

Test plan

• `grep` confirms no remaining CF / Cloudflare / `CF_ACCESS` / `cloudflared` references in `docs/` or `README.md`.
• No inbound links to the deleted section anywhere in the repo.
• Auto-generated `docs/kh_*.md` files (cobra) do not reference CF — no command help text mentions it.
• Reviewer sanity-check: `kh auth login` against a CF-gated host still works with `CF_AUTHORIZATION` set.