Complete NavCoin port forwarding guide

[ChuanWeis]

Description

Completes the existing draft article for opening NavCoin wallet firewall ports. The article now covers:

• when port forwarding is useful
• checking wallet active connections first
• reserving a local IP address
• forwarding port 44440 on the router
• allowing the same port through the computer firewall
• restarting NavCoin Core and checking connections again
• where to go next if the wallet still has zero connections

Issues Resolved

Fixes #4

Preview Links

A Netlify preview should be available from the pull request checks once the deploy is created.

Expected generated path from local build:

https://deploy-preview--navcoin-knowledgebase.netlify.com/general-support/how-to-open-firewall-ports-for-navcoin/

Checklist

• Have you assigned/claimed the issues you've resolved in this pull request?
• Have the issues been moved to the QA column of the NavCoin Websites project?
• Have you checked there are no merge conflicts?
• Have you tested the changes work across both mobile and desktop?

Validation performed:

• git diff --check
• static Markdown/front matter/link checks
• Hugo Extended v0.55.6 build passed and generated /general-support/how-to-open-firewall-ports-for-navcoin/ with the expected title and port 44440 content

Note: Hugo Extended v0.161.1 currently exposes existing theme/content compatibility issues in this repository, so I used v0.55.6 for the local build check.