Update/Improve RFsim documentation

- Add TOC
- Better describe what RFsim does
- Remove misleading, wrong, unnecessary information
  * It is futile to mention --phy-test or other modes, RFsim works with
    all
  * Explaining how to ping is not the task of the RFsim documentation
    (that is what tutorials are for)
  * Remove useless/outdated build log information
- Add some more information on which flags to run
- Improve formatting, and minor clarifications

doc/RUNMODEM.md

@@ -26,7 +26,7 @@ It is planned to enhance this simulator with the following functionalities:
 
 - Support for multiple eNodeB's or gNodeB's for hand-over tests
 
-This is an easy use-case to setup and test, as no specific hardware is required. The [rfsimulator page](../radio/rfsimulator/README.md ) contains the detailed documentation.
+This is an easy use-case to setup and test, as no specific hardware is required. The [rfsimulator page](../radio/rfsimulator/README.md) contains the detailed documentation.
 
 ## L2 nFAPI Simulator