MT#55897 Add the RTPEngine's usage synopsis to the RTD

Start using the synopsis in the RTD hierarchy of files.

Change-Id: I6979bd6f1af46b8db41944e021ef7533c7e28126

docs/index.rst

@@ -14,6 +14,8 @@ Currently the only supported platform is GNU/Linux.
    :maxdepth: 4
 
    overview
+   rtpengine man <rtpengine>
+   rtpengine-recording man <rtpengine-recording>
    architecture
    tests
    troubleshooting