Instructions don't address key conversion

**Motivation**
I am trying to install Buzz on a VPS / cloud machine. I generate 2 key pairs on that machine in hex format, for the owner and the relay. I put the relay secret key and the owner pub key in the .env. Then I start the app on my laptop and it wants a secret key in nsec format. I don't have that, because the instructions give me a hex format key pair. Nowhere does it say how I should convert the owner secret key to nsec format. So I am stuck.

**Proposed solution**
Give the instruction how to convert my hex secret key of the owner to an nsec secret key that the app accepts

Start from the installation and key-setup instructions referenced by the issue, then trace where the owner secret key format is described. Document how to convert the owner hex secret key to the nsec format accepted by the app, and verify that the updated instructions cover the VPS/cloud setup and laptop startup flow.

Written by the indexing model from the issue text.