Add debugging/testing public reachability guide with Kubo

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What

Add a new debugging guide for public reachability based on the new command introduced. by ipfs/kubo#11184

Why

Public reachability is the bread and butter for IPFS nodes and necessary for the following things to work:

• Direct data fetching: Other nodes can fetch data directly from your node without NAT hole punching.
• Browser access: Web browsers can connect to your node directly for content retrieval.
• DHT participation: Your node can act as a DHT server, helping to maintain the distributed hash table and making content routing more robust.

Up until now, we didn't have any way to debug public reachability. ipfs/kubo#11184 introduces a new command to Kubo which should help with this.

• cc Add NAT troubleshooting to documentation kubo#6952