watchtower guide: use maintained fork (upstream archived)

[OCharnyshevich]

The watchtower guide points at containrrr/watchtower, which was archived (read-only) on 2025-12-17 and no longer works on current Docker Engine (28/29) — following the guide as-is now produces a client version 1.25 is too old crash loop.

This updates the guide to the actively maintained, drop-in fork openserbia/watchtower:

• GitHub + DockerHub links → the fork
• compose image: → openserbia/watchtower:latest
• a short note explaining the original was archived (with a pointer to it and to other maintained forks like nicholas-fedor/watchtower)

Behavior, flags, labels, and .env config are unchanged — it's just a different image name.

Disclosure: I maintain the fork.