Add hint for /oem/harvester.config

[votdev]

I think it makes sense and would be helpful to include this information in the documentation.

[github-actions]

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[jillian-maroket]

Changed the admonition type and tightened the wording

[tserong]

Good call out.

I should mention though that there's one exception to this. If the user runs harvester-installer generate-network-config it will re-create the NM connection profiles based on the contents of /oem/harvester.config - see https://docs.harvesterhci.io/v1.8/upgrade/v1-6-x-to-v1-7-x#migration-from-wicked-to-networkmanager. That's a way of re-setting your NM connection profiles back to what they were originally if you happen to screw them up somehow. But, this is not something that Harvester will ever automatically do.