Create sequence diagrams / usage instructions for sdsread/write

[ReinhardKeil]

For the section https://arm-software.github.io/SDS-Framework/main/theory/.

1. There should be sequence diagrams that explain how the sdsio operations are protected for multi-threading.

   • what does a user need to consider to implement own sdsio functions?

2. Clear usage instructions for multi-threading applications.

   • can sdsclose called in the middle of a sdswrite/read operation?
   • should sdsopen/write/read executed from the same thread?
     • handling of open/close here https://github.com/Arm-Examples/SDS-Examples/blob/main/Hardware/ModelTest/ei_main.cpp#L76

3. Document maximum execution timing of Open Read/Write Close

[ReinhardKeil]

SDSIO Server commands are covered here https://github.com/ARM-software/SDS-Framework/blob/main/sds/source/sdsio/serial/usart/sdsio_serial.c

[ReinhardKeil]

@DavidLesnjak I believe this is complete. If so, please close it.