docs: add TOTP feature to README

[threehappypenguins]

Closes #716

Summary

Adds optional TOTP two-factor authentication to the Restreamer Features list.

No runtime or bundle code changes — documentation only. The feature itself lives in the companion core and restreamer-ui PRs.

Full TOTP documentation (enrollment, disable, recovery, trusted devices) is in the core PR as docs/TOTP.md. That file is intended for publication on docs.datarhei.com/restreamer (GitBook), not the repo README. Could maintainers please:

• Add it under Manual → Authorization (enrollment and settings, matching Settings → Authentication in the UI)
• Add a User Guide for recovery, e.g. “How do I reset TOTP if I lost my authenticator?” — alongside the existing password guide

Test plan

• README renders correctly on GitHub
• Features list includes the TOTP bullet