docs(SLIM): configuration reference (#230)

* docs(SLIM): configuration reference

Signed-off-by: Mauro Sardara <msardara@cisco.com>

* chore: formatting

Signed-off-by: Aron Kerekes <arkereke@cisco.com>

---------

Signed-off-by: Mauro Sardara <msardara@cisco.com>
Signed-off-by: Aron Kerekes <arkereke@cisco.com>
Co-authored-by: Aron Kerekes <arkereke@cisco.com>

Author: msardara

docs/messaging/.index

@@ -1,6 +1,8 @@
 nav:
  - Secure Low-Latency Interactive Messaging (SLIM): slim-core.md
- - Getting Started with SLIM: slim-howto.md
+ - Getting Started with SLIM:
+    - Getting Started: slim-howto.md
+    - Configuration Reference: slim-data-plane-config.md
  - SLIM Messaging Layer: slim-data-plane.md
  - SLIM Controller: slim-controller.md
  - SLIM Group Management: slim-group.md