docs: add schedule article guide

In this guide we will see how plan article publication base on the "Draft system" guide.
Usage of CRON functions and services of API to fetch and update entries.

docs/3.0.0-beta.x/guides/schedule-article.md

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+# Schedule article
+
+This guide will explain how to create an article schedule system.
+
+## Introduction
+
+This guide will use this [draft system](.draft.md) guide. So first read it to understand the draft/published status and the Article API that we will use in this guide.
+
+What we want here is to be able to set a publication date for an article, and at this date, switch the draft status to published.
+
+## Example
+
+For this example, we will have to add a `published_at` attribute to the Article Content Type.
+
+- Click on the Content Type Builder link in the left menu
+- Select the Article Content Type
+- Add another field
+  - `date` attribute named `published_at` with `datetime` type
+
+And add some data with different dates and status to be able to see a plublication happen.
+Some entries need to have `draft` status and a `published_at` date lower that your current date.
+
+The goal will be to verify every minute if there is `draft` articles that have a `published_at` lower that the current date.
+
+## Create a CRON task
+
+To execute a function every minutes, we will use CRON tasks.
+
+Here is the [full documentation](../concepts/configurations.md#cron-tasks) of this feature.
+
+**Path —** `./config/functions/cron.js`
+
+```js
+module.exports = {
+  '*/1 * * * *': () => {
+    console.log('1 minute later');
+  },
+};
+```
+
+<<<<<<< HEAD
+Make sure the enabled cron config is set to true in ./config/environments/**/server.json file.
+
+=======
+>>>>>>> ecb9ac901bb0e8e04a0a1f54e9c1ffce293bb116
+## Business login
+
+Now we can start to write the code that will fetch all draft Articles that have a `published_at` lower than the current date.
+
+Then we will update the `status` of all these articles to `published`.
+
+**Path —** `./config/functions/cron.js`
+
+```js
+module.exports = {
+<<<<<<< HEAD
+  '*/1 * * * *': async () => {
+=======
+  '*/1 * * * *': () => {
+>>>>>>> ecb9ac901bb0e8e04a0a1f54e9c1ffce293bb116
+    // fetch articles to publish
+    const draftArticleToPublish = await strapi.api.article.services.article.find({
+      status: 'draft',
+      published_at_lt: new Date()
+    });
+
+    // update status of articles
+    draftArticleToPublish.forEach(async (article) => {
+      await strapi.api.article.services.article.update(
+        {id: article.id},
+        {status: 'published'}
+      );
+    });
+  },
+};
+```
+
+And tada!
+
+