docs(docs/admin/users): document login_type=none deprecation and password conversion - #27292

🤖 This PR was written by Coder Agents on behalf of Jake Howell.

Docs follow-up for #26851 (DEVEX-226). Stacked on that branch so the diff is docs-only; rebase onto main once #26851 lands.

What this does

Updates docs/admin/users/headless-auth.md to document the deprecation of login_type=none and the conversion of existing accounts:

• Creation is rejected. Documents that coder users create --login-type none (and the deprecated --disable-login) now error unless --service-account is passed, and that POST /users returns 400 for login_type: "none" without a service account. Uses the exact error strings from cli/usercreate.go and coderd/users.go.
• Premium / OSS split. Notes that service accounts require Premium and points OSS deployments at regular users (password/GitHub/OIDC), linking the existing CI/CD tutorial.
• Upgrade behavior. Documents that upgrading converts existing non-system login_type=none users to password login, preserving their email and existing API tokens (matching fix!: deprecate login_type=none, convert existing users to password login #26851). An admin must reset the password before web-UI login; token automation is unaffected. One-way conversion.

No new pages, so no manifest.json change.

Verification

• Error strings copied verbatim from the code in fix!: deprecate login_type=none, convert existing users to password login #26851 (cli/usercreate.go, coderd/users.go); upgrade behavior matches 000548_legacy_none_login_to_password.up.sql.
• make lint/emdash, markdownlint-cli2, and vale all pass on the edited page.