Documentation: system: cu: document the cu serial terminal

[JorgeGzm]

Summary

The cu application had no documentation (the page was an empty stub).
This PR implements it:

• Document the cu command: description, options, escape sequences,
  dependencies and limitations, plus a "Bridging two serial ports"
  section showing how to bind two ports to each other from the shell
  (bidirectional or unidirectional, including ports at different
  baud rates).
• Add an rs485 configuration to the linum-stm32h753bi board (nsh +
  the two on-board RS-485 transceivers + cu), used to test the
  documented commands on real hardware.
• The tests performed on the board are documented in the
  linum-stm32h753bi board documentation: bridging the console to an
  RS-485 bus with cu, binding the two buses to each other, and a
  loopback self-test between the two transceivers.

Impact

• User: cu is now documented; linum-stm32h753bi gets a
  ready-to-use RS-485 configuration.
• Build: none, documentation and new optional defconfig only.
• Hardware: linum-stm32h753bi only.
• Documentation: yes, this PR.
• Compatibility: none.

Testing

All commands tested on the linum-stm32h753bi board (RS-485 loopback
A-A/B-B between the two on-board transceivers):

nsh> cat /dev/ttyS1 > /dev/ttyS2 &
nsh> cat /dev/ttyS2 > /dev/ttyS1 &
nsh> cu -l /dev/ttyS1 -s 115200
[typed characters come back through the bridge]
~.

• tools/configure.sh linum-stm32h753bi:rs485 from distclean + full
  build (GCC with -Werror: no warnings).
• sphinx-build: no warnings on the changed pages.
• tools/checkpatch.sh: all checks pass.

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb