Fix #1228 - Separate DNS docs and add notes on MX, FCrDNS/SPF/DKIM/DMARC #927

Job	Run time
build-docker (internet.nl, docker/Dockerfile, app)	1m 8s
build-docker (unbound, docker/Dockerfile, unbound)	52s
build-docker (linttest, docker/Dockerfile, linttest)	56s
build-docker (test-runner, docker/test-runner.Dockerfile)	1m 1s
build-docker (webserver, docker/webserver.Dockerfile)	1m 18s
build-docker (grafana, docker/grafana.Dockerfile)	53s
build-docker (cron, docker/cron.Dockerfile)	1m 7s
integration-test	3m 9s
docs	1s
development-environment-test	2m 12s
lint/check	52s
test	2m 7s
	15m 36s

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