doc/userguide: display version on front page

When viewing the docs online at Readthedocs, or similar it might be
immediately apparent what version of the documentation is being
displayed. Display the version on the first line before the table of
contents to make it clear.

doc/userguide/index.rst

@@ -1,6 +1,8 @@
 Suricata User Guide
 ===================
 
+This is the documentation for Suricata |version|.
+
 .. toctree::
    :numbered:
    :maxdepth: 2