community/ntpsec: improve

• Assume maintainership
• Add init script (adapted from https://gitlab.com/NTPsec/ntpsec/-/blob/master/etc/ntpd.service and busybox's ntpd service file)
• Add sample configuration file (taken from https://gitlab.com/NTPsec/ntpsec/-/blob/master/etc/ntp.d/default.conf)
• Build docs with asciidoctor (upstream recommends it rather than using asciidoc)
• Make sed command a bit more readable