Drop md extension from runbook URLs

roles/prometheus/templates/rules/alerts_acme.conf.yml

roles/prometheus/templates/rules/alerts_backup.conf.yml

roles/prometheus/templates/rules/alerts_base.conf.yml

roles/prometheus/templates/rules/alerts_cpu.conf.yml

roles/prometheus/templates/rules/alerts_disk.conf.yml

roles/prometheus/templates/rules/alerts_mysql.conf.yml

roles/prometheus/templates/rules/alerts_net.conf.yml

roles/prometheus/templates/rules/alerts_nginx.conf.yml

roles/prometheus/templates/rules/alerts_prometheus.conf.yml

roles/prometheus/templates/rules/alerts_services.conf.yml

roles/prometheus/templates/rules/alerts_ssl_probes.conf.yml

roles/prometheus/templates/rules/alerts_syslog.conf.yml

.../prometheus/templates/rules/alerts_system_health.conf.yml

roles/prometheus/templates/rules/alerts_systemd.conf.yml