getting-started: register-device: add tag parameter for first target

[mike-scott]

Previously, our "fast first target" artifacts were built from the "main" branch of the LmP and would auto-register using the "main" tag.

With the latest changes to our LmP workflow, we now have LTS branch names such as "scarthgap" and also a "main" branch representing upstream Yocto / OE development.

The artifacts used for LmP version releases will most likely come from an LTS branch, and will attempt to register using that branch name by default (IE: "scarthgap" tag).

The device should be registered to follow the "main" tag to match the 2nd target of a newly created Factory. So let's add the "-t main" parameter to our docs.

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[kprosise]

Lgtm