T3871: bind interface names to hardware slots instead of hw-id (#2246)

* docs: T3871: describe how interface names are assigned

Names are no longer derived from the hw-id node. They are bound to the slot
the hardware occupies and recorded in a mapping file, so document what
decides a name, what happens when a card is replaced or added, and how to
rename an interface.

The troubleshooting page said to pin a MAC with hw-id when interfaces were
renamed after a VM migration or a clone. That node no longer exists and the
situation it describes no longer arises, so replace it with a pointer to the
new section and a note on where to look when an interface really is missing.

* docs: T3871: drop hw-id from the example output

The node is gone from the CLI, so example configuration dumps which still
show it no longer match what a reader sees on their own system. Remove it
from the shown output only; no prose changes. The comparison shown on the
testing page is rewritten rather than trimmed, as deleting the lines would
have left an edit level behind with nothing under it.

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