usage: getting-started: add a section for portable devices

[ES-Alexander]

A convenient but under-represented device class for Cockpit is portable/handheld/mobile devices.

#3 is a start on this (for the steamdeck), but we should also specifically cover tablets/phones, and likely other handheld devices as well.

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• Suggested by @patrickelectric
• @vshie has mentioned such devices may be more likely to use wifi connections than direct tethers, which would be nice to factor in