Prefer NAT Taversal rather than holepunching

[flub]

We decided to standardise on the term "NAT Traversal" instead of "holepunching" and will be slowly moving all our comments, docs etc to that naming. We are aware that you might still need to perform holepunching though firewalls, even when there is no NAT involved. But decided that it was the more commonly used term in technical discussions and standardisation efforts.

The https://docs.iroh.computer/concepts/holepunching page centres entirely on the "holepunching" term, it should be reworked to use NAT Traversal as well.