[Docs] Extend data checks docs and add screenshot

docs/admin/33_data_checks.rst

+.. _core-data-checks:
+
 Data checks
-================
+===========
+
+Data checks are AlekSIS' mechanism for highlighting issues with the
+contents of the database. These checks are not of a technical nature,
+but strictly concern the contextual integrity of the data stored.
+
+
+Verify data checks
+------------------
 
-TBA.
+In the menu under ``Admin → Data checks``, the status of all known
+checks can be verified.
 
-.. _data-checks:
+.. image:: ../_static/data_checks.png
+   :width: 100%
+   :alt: Data check overview
+
+The first card shows the current global check state. If any data checks
+reported issues, they will be listed here. In that case, administrators can
+choose between options provided by the data checks to resolve the issues.
+
+.. note::
+   Details about the checks and solve options are described in the
+   respective chapters of the manual.
 
 Configure notifications
 -----------------------
 
 In the ``General`` tab of the configuration interface, you can configure
-email notifications for problems detected by the data checks
+email notifications for problems detected by the data checks.
 
 * General
 
   * Send emails if data checks detect problems: Enable email notifications
   * Email recipients for data checks problem emails: Choose recipient persons
   * Email recipient groups for data checks problem emails: Choose recipient groups
+
+Data checks normally run once per hour, and if notifications are enabled, results
+will be mailed to the selected recipients if problems are detected.