Provide logback backend configuration (example) by default in the Karaf distribution

I tried to switch the log backend from log4j2 to logback. I can start the karaf, but the log file data/log/karaf.log contains only very limited log entries (see below).

```
Feb 24, 2026 9:49:24 AM org.apache.karaf.main.Main launch
INFO: Installing and starting initial bundles
Feb 24, 2026 9:49:24 AM org.apache.karaf.main.Main launch
INFO: All initial bundles installed and set to start
Feb 24, 2026 9:49:24 AM org.apache.karaf.main.lock.SimpleFileLock lock
INFO: Trying to lock .../apache-karaf-minimal-4.4.9/lock
Feb 24, 2026 9:49:24 AM org.apache.karaf.main.lock.SimpleFileLock lock
INFO: Lock acquired
Feb 24, 2026 9:49:24 AM org.apache.karaf.main.Main$KarafLockCallback lockAcquired
INFO: Lock acquired. Setting startlevel to 100
```

Steps to reproduce:

1. In file `etc/org.apache.karaf.features.cfg`: change `framework` to `framework-logback`
2. In file `etc/startup.propertie`: change `pax-logging-log4j2` to `pax-logging-logback`
3. Start karaf: `bin/karaf`