doc: add alert documentation

Add "Alerting" page to "Monitoring" documentation section. This page
contains the link to grafana-dashboard alerts.yml [1] example,
example rules (same as in the file) with comments and a short section
about external monitoring.

1. https://github.com/tarantool/grafana-dashboard/tree/master/example_cluster/prometheus/alerts.yml

Closes #64

Author: DifferentialOrange