config/network/firewall.md: needs some rework/clarifications.

We currently have here two sections "Applying rules at boot" and "Applying rules at runtime" both are ambiguous.
Both of them work at boot and at runtime, they are two alternative methods, with the one "Applying rules at boot" being clearly the better solution because they don't need a fake service and they make sure firewall rules are loaded before network services are up.

• Change the headlines for "Applying rules at X".
• Make clear that they are two ways of loading rules at boot and at runtime, make clear that core-services/rc.local is superior.
• Document how to reload rules with the fake service, sv can be used and with the better core-services or rc.local method the nftables (or iptables) command can be used.

config/network/firewall.md を開き、現在「Applying rules at boot」および「Applying rules at runtime」というタイトルになっているセクションを比較してください。これらが起動時とランタイムにルールを読み込むための代替方法であることを明確にし、core-services/rc.local が推奨される理由を説明し、sv または nftables/iptables コマンドを使った再読み込みについて記述してください。更新後のセクション見出しと手順によって、曖昧さが解消されるようにしてください。