feat(onboarding): show friendly log with path to default admin password

[inder02]

This PR adds a log message to inform users where to find the initial admin password file
when the server starts. This improves onboarding experience, especially when running in containers.

Inspired by how Jenkins shows the password file path in logs.

[inder02]

Hi maintainers 👋

I'm new to the DevOps field and currently learning tools like Jenkins and Nexus. While working with Nexus, I noticed that it doesn't clearly guide new users to the location of the initial admin password when running in a container — something Jenkins does really well during onboarding.

This small usability issue stood out to me because I faced it personally while learning Nexus. So, I decided to try solving it — and that’s what this PR does! 🙌

This is also my first open-source contribution, and I’m very excited (and a bit nervous 😄). I'm open to any suggestions or improvements you might have.

Thanks a lot for reviewing and maintaining this awesome project! 🚀