Add info about blocking x_tables module when using nf_tables

According to website https://wiki.nftables.org/wiki-nftables/index.php/Moving_from_iptables_to_nftables you should not use x_tables (iptables) and nf_tables (nftables) kernel modules at the same times. At the bottom of the page there is this:

Beware of using both the nft and the legacy tools at the same time. That means using both x_tables and nf_tables kernel subsystems at the same time, and could lead to unexpected results.

I suggest to add info about blocking module x_tables to the section about nftables on the page https://docs.voidlinux.org/config/network/firewalls.html .

On my personal computer with void linux I created file /etc/modprobe.d/iptables-blacklist.conf with content:

blacklist x_tables
install x_tables /bin/false

Seems to block x_tables effectively.

Start with the nftables section of the mdBook source that renders as docs.voidlinux.org/config/network/firewalls.html, then read the linked nftables migration guidance. Done means the page explains the x_tables and nf_tables conflict and includes the proposed modprobe configuration, with the documentation build or rendered page checked.