vaultwarden: add tab about how to enable the admin panel

[MichaIng]

MichaIng/DietPi#8237

1. Create (Argon2ID) hash via CLI /opt/vaultwarden/vaultwarden hash: Enter and confirm a strong password, and the command returns the hash.
2. Add ADMIN_TOKEN to /mnt/dietpi_userdata/vaultwarden/vaultwarden.env, or admin_token to the config.json. Need to check path. And we should apply 0640 mode to /mnt/dietpi_userdata/vaultwarden/vaultwarden.env our end, when suggesting this. config.json would be better, to not expose the password to systemd at all.
3. Access via https://<your.IP>:8001/admin.

EDIT: Word-read bit is removed now precautionary: MichaIng/DietPi@60d0bcc