Add cron_jobs.md.

Author: DavidLambauer

cron_jobs.md

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+# Cron Jobs XML Reference Documentation
+
+## Introduction
+
+This reference documentation provides an in-depth explanation of the cron_jobs.xml file in Magento 2. Cron jobs are
+scheduled tasks that are executed at predefined intervals. The cron_jobs.xml file allows you to configure and manage
+these cron jobs in Magento 2.
+
+## File Location and Structure
+
+The cron_jobs.xml file is located in the `app/code/*Vendor*/*Module*/etc` folder of your Magento installation. It
+follows the XML file format and defines the configuration for cron jobs.
+
+Here is an example of the file structure:
+
+```xml
+<?xml version="1.0"?>
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+        xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Cron:etc/crontab.xsd">
+    <group id="default">
+        <job name="job_name" instance="*Vendor*\*Module*\Cron\*CronClass*" method="execute">
+            <schedule>*schedule_expression*</schedule>
+        </job>
+    </group>
+</config>
+```
+
+## File Components
+
+The cron_jobs.xml file consists of the following components:
+
+### 1. `config` Root Element
+
+The `config` root element encloses the entire XML configuration. It includes the `xmlns:xsi` attribute, which specifies
+the XML namespace for the `xsi` prefix, and the `xsi:noNamespaceSchemaLocation` attribute, which points to the cron jobs
+XSD schema location.
+
+### 2. `group` Element
+
+The `group` element groups related cron jobs together. It has a required `id` attribute that identifies the group.
+
+### 3. `job` Element
+
+The `job` element defines a single cron job within a group. It has the following attributes:
+
+- `name`: Specifies the unique name of the cron job.
+- `instance`: Specifies the fully qualified class name of the cron job's implementation class.
+- `method`: Specifies the method of the cron job's implementation class that should be executed.
+
+### 4. `schedule` Element
+
+The `schedule` element specifies the cron expression or schedule for the job. It defines when the cron job should be
+executed. The `*schedule_expression*` should follow the cron syntax.
+
+## Example
+
+Let's consider an example to illustrate the usage of the cron_jobs.xml file. Assume we have a module
+named `Vendor_Module` with a cron job that needs to run every hour.
+
+1. Create the `cron_jobs.xml` file in the module's `etc` folder with the following content:
+
+```xml
+<?xml version="1.0"?>
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+        xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Cron:etc/crontab.xsd">
+    <group id="default">
+        <job name="vendor_module_cron" instance="Vendor\Module\Cron\MyCronClass" method="execute">
+            <schedule>0 * * * *</schedule>
+        </job>
+    </group>
+</config>
+```
+
+2. In the above example, the `group` element has an `id` attribute set to `default`. The `job` element has a `name`
+   attribute set to `vendor_module_cron`, an `instance` attribute set to `Vendor\Module\Cron\MyCronClass`, and
+   a `method` attribute set to `execute`. The `schedule` element specifies that the cron job should run every hour at
+   the 0th minute.
+
+3. In your PHP code, create the implementation class `MyCronClass` under `Vendor\Module\Cron`:
+
+```php
+<?php
+namespace Vendor\Module\Cron;
+
+class MyCronClass
+{
+    public function execute()
+    {
+        // Perform the required cron job tasks here
+    }
+}
+```
+
+4. With the above configuration and code, the `execute()` method of the `MyCronClass` class will be executed every hour.
+
+## Conclusion
+
+The cron_jobs.xml file allows you to configure and manage cron jobs in Magento 2. By following the structure and syntax
+described in this documentation, you can define cron jobs and specify their schedules. Remember to provide the necessary
+implementation class and method for each cron job.
+