Update rpm-systems.md to include linking the sensor under the orb user

[velosol]

If you go to link the sensor to an Orb account you need to do it under the orb user, not the currently logged in user who isn't going to be the user context when systemd starts the sensor.

Pull Request

What changes have you made?

Added a section for RPM-based distros showing how to properly link the systemd-run sensor to your Orb account.

Why are these changes helpful?

If someone naively runs orb link after installing the sensor the link info will be stored under that user, not the orb user that systemd uses to run the sensor at boot. This leads to a weird situation where the sensor shows online if the logged in user that did the linking runs orb sensor but not with the systemd-run orb sensor.

Related issues or considerations

May be useful in other Linux distro pages or as part of the overarching Linux setup page but I don't have other systems to test at the moment so will leave that choice up to maintainers (and happy to put this on another or other pages as desired).

Checklist

• Documentation is clear and understandable
• No broken links or images
• Follows project style and formatting
• [NA] If scripts were modified/included, they have been tested and work as expected

Additional notes

Thank you to Wayne who ID'ed what I had done wrong and clearly explained what I needed to do to fix it. Figured it would be better in the docs as that's the first place I went before reaching out to support.

Please wait for feedback from the maintainers before merging.