doc/userguide: add a known issues page

The idea of this page is to track known issues on a release branch
that we do not plan to fix.

(cherry picked from commit a4f722fca0)

doc/userguide/Makefile.am

@@ -9,6 +9,7 @@ EXTRA_DIST = \
 	devguide \
 	file-extraction \
 	index.rst \
+	known-issues.rst \
 	upgrade \
 	upgrade.rst \
 	initscripts.rst \

doc/userguide/index.rst

@@ -35,3 +35,4 @@ This is the documentation for Suricata |version|.
    licenses/index.rst
    devguide/index.rst
    verifying-source-files
+   known-issues

doc/userguide/known-issues.rst

@@ -0,0 +1,9 @@
+Known Issues
+############
+
+The following is a list of known issues in Suricata 7.0.
+
+* When an IKE message contains multiple attributes with the same key,
+  only the first will be logged to avoid producing invalid JSON, or
+  introducing a logging format change. See
+  https://redmine.openinfosecfoundation.org/issues/7923.