✨ added healthchecks.io service integration

docs/applications/healthchecks.io.md

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+# Healthchecks.io
+
+Homepage: [https://healthchecks.io/](https://healthchecks.io/)
+
+A simple cronjob that uses `curl` to ping a given endpoint on the `healthchecks.io` servers. You can choose how often it should ping the endpoint, and what happens when it doesn't. Email/Slack/Telegram and many more services can be integrated.
+
+## Usage
+
+Create your own project on [https://healthchecks.io/](https://healthchecks.io/), and set both the time between pings and the grace time. Set your prefered integration such as email.
+
+Set `healthchecks_enabled: true` in your `inventories/<your_inventory>/nas.yml` file, and if your time between pings is different than the default `healthchecks_ping_minutes`, change it. Finally, set your ping url in the `healthchecks_url` variable.

roles/healthchecks.io/defaults/main.yml

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+---
+healthchecks_enabled: false
+healthchecks_ping_minutes: 15
+
+healthchecks_url: ""

roles/healthchecks.io/tasks/main.yml

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+---
+- name: Add healthchecks.io cronjob
+  cron:
+    name: healthchecks.io
+    minute: "*/{{ healthchecks_ping_minutes }}"
+    user: root
+    cron_file: /etc/crontab
+    job: "curl -m 10 --retry 5 {{ healthchecks_url }}"