Add securitytxt to the list of plugins (#949)

Part of request #26734: Add basic support of security.txt file (RFC 9116)

languages/en/administration-guide/system-administration/securitytxt.rst

@@ -0,0 +1,16 @@
+..  _securitytxt:
+
+Support of security.txt file
+============================
+
+A `security.txt <https://securitytxt.org/>`_ file can be deployed easily with the ``securitytxt`` plugin.
+This file makes possible for security researchers to find a point of contact to report security issues
+they might have found on your instance.
+
+In order to make it work you need :ref:`to install the package and activate the plugin in the site administration <install-plugins>`.
+The primary point of contact must be defined by setting a value to the ``security_txt_primary_contact`` parameter. The value must
+follow the URI syntax, for more information see `RFC 9116 section 2.5.3 <https://www.rfc-editor.org/rfc/rfc9116.html#name-contact>`_.
+
+.. code-block:: bash
+
+    $ tuleap config-set security_txt_primary_contact mailto:security@example.com

languages/en/installation-guide/install-plugins.rst

@@ -127,6 +127,7 @@ Authentication and permissions:
 Administration:
 
 * **archivedeleteditems**: When something is deleted, move it into a dedicated place for archiving purpose
+* :ref:`securitytxt <securitytxt>`: Deploy a `security.txt <https://securitytxt.org/>`_ (`RFC 9116 <https://www.rfc-editor.org/rfc/rfc9116.html>`_) file
 * **statistics**: Compute stats about platform
 * :ref:`prometheus_metrics <admin_monitoring_with_prometheus>`: Expose metrics to Prometheus about Tuleap usage