handbook/resources: remove references to bsd.network

As requested by bsd.network admins, the Mastodon-based social community
is a private group, not a public resource. While many BSD aligned people
may use it, this also ensures the privacy of those users is respected.

Reported by: Peter Hessler <admin@bsd.network>
Approved by: debdrup
Sponsored by: SkunkWerks, GmbH
Differential Revision: https://reviews.freebsd.org/D35126