Merge branch 'internal-cron-docs' into 'master'

Make internal cron docs

Closes #229864

See merge request gitlab-org/gitlab!38961

doc/topics/cron/index.md

+# Cron
+
+Cron syntax is used to schedule when jobs should run.
+
+You may need to use a cron syntax string to
+[trigger nightly pipelines](../../ci/triggers/README.md#using-cron-to-trigger-nightly-pipelines),
+create a [pipeline schedule](../../api/pipeline_schedules.md#create-a-new-pipeline-schedule),
+or to prevent unintentional releases by setting a
+[deploy freeze](../../user/project/releases/index.md#prevent-unintentional-releases-by-setting-a-deploy-freeze).
+
+## Cron syntax
+
+Cron scheduling uses a series of five numbers, separated by spaces:
+
+```plaintext
+# ┌───────────── minute (0 - 59)
+# │ ┌───────────── hour (0 - 23)
+# │ │ ┌───────────── day of the month (1 - 31)
+# │ │ │ ┌───────────── month (1 - 12)
+# │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday)
+# │ │ │ │ │
+# │ │ │ │ │
+# │ │ │ │ │
+# * * * * * <command to execute>
+```
+
+[Source: [Wikipedia](https://en.wikipedia.org/wiki/Cron)]
+
+In cron syntax, the asterisk (`*`) means 'every,' so the following cron strings
+are valid:
+
+- Run once an hour at the beginning of the hour: `0 * * * *`
+- Run once a day at midnight: `0 0 * * *`
+- Run once a week at midnight on Sunday morning: `0 0 * * 0`
+- Run once a month at midnight of the first day of the month: `0 0 1 * *`
+- Run once a year at midnight of 1 January: `0 0 1 1 *`
+
+For complete cron documentation, refer to the
+[crontab(5) — Linux manual page](https://man7.org/linux/man-pages/man5/crontab.5.html).
+This documentation is accessible offline by entering `man 5 crontab` in a Linux or MacOS
+terminal.
+
+## Cron examples
+
+```plaintext
+# Run at 7:00pm every day:
+0 19 * * *
+
+# Run every minute on the 10th of June:
+* * 3 6 *
+
+# Run at 06:30 every Friday:
+30 6 * * 5
+```
+
+More examples of how to write a cron schedule can be found at
+[crontab.guru](https://crontab.guru/examples.html).
+
+## How GitLab parses cron syntax strings
+
+GitLab uses [fugit](https://github.com/floraison/fugit) to parse cron syntax
+strings on the server and [cron-validate](https://github.com/Airfooox/cron-validate)
+to validate cron syntax in the browser.

doc/topics/index.md

@@ -10,6 +10,7 @@ tutorials, technical overviews, blog posts) and videos.
 - [Auto DevOps](autodevops/index.md)
 - [Authentication](authentication/index.md)
 - [Continuous Integration (GitLab CI/CD)](../ci/README.md)
+- [Cron](cron/index.md)
 - [Git](git/index.md)
 - [GitLab Flow](gitlab_flow.md)
 - [GitLab Installation](../install/README.md)