devguide: add chapter and short intro to libsuricata

With this, we intend to make more users aware of this use case, and that
we are working towards this.

Related to
Task #2693

doc/userguide/devguide/index.rst

@@ -8,3 +8,4 @@ Suricata Developer Guide
    contributing/index.rst
    internals/index.rst
    extending/index.rst
+   libsuricata/index.rst

doc/userguide/devguide/libsuricata/index.rst

@@ -0,0 +1,14 @@
+.. _libsuricata:
+
+LibSuricata
+===========
+
+Using Suricata as a Library
+---------------------------
+
+The ability to turn Suricata into a library that can be utilized in other tools
+is currently a work in progress, tracked by Redmine Ticket #2693:
+https://redmine.openinfosecfoundation.org/issues/2693.
+
+A related work are Suricata plugins, also in progress and tracked by Redmine
+Ticket #4101: https://redmine.openinfosecfoundation.org/issues/4101.