Updated and enhanced channel simulation documentation

- centralized channel simulation documentation in channel_simulation.md
- removed redundant documentation repeated in multipled .md files
- updated links
- removed obsolete information
- introduced mermaid flowchart for rfsim
- added missing information about available options and usage
- deleted:    ../../../openair1/SIMULATION/TOOLS/DOC/devusage.md
- deleted:    ../../../openair1/SIMULATION/TOOLS/DOC/rtusage.md
- deleted:    ../../../openair1/SIMULATION/TOOLS/DOC/arch.md

related to issue #739