Migrate the dns setup page into its own page

This moves the dns setup out of the networking page into its own section. That also fixes and updates the /etc/hosts inlined [2] (which will be redirected to that new page as well).

It's currently in the same page which describes the network configuration [1]. And i'm having a hard time deciding whether i want to migrate that one or not. So i keep an easier path and move that one part instead.

• [1] https://intranet.softwareheritage.org/wiki/Network_configuration#Dns_servers

• [2] https://intranet.softwareheritage.org/wiki/Network_configuration/etc/hosts

Related to swh/infra/sysadm-environment#3154 (closed)

Test Plan

make -C sysadm html

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Migrated from D6568 (view on Phabricator)