Documentation: nsh: document the top command

[JorgeGzm]

Summary

The nsh top command exists since 2024 but was never documented.

• Add a top section to the NSH commands documentation page
  (Documentation/applications/nsh/commands.rst): syntax, options,
  example output and configuration dependencies.
• Add a top usage example to the linum-stm32h753bi board
  documentation.
• Enable the command in the linum-stm32h753bi:nsh defconfig: procfs,
  SCHED_CPULOAD, STACK_COLORATION and TASK_NAME_SIZE, so top
  and ps are fully functional out of the box. This also configures a
  dedicated interrupt stack and enlarges the IDLE/init task stacks:
  with SCHED_CPULOAD the per-tick accounting runs in interrupt
  context, and with ARCH_INTERRUPTSTACK=0 it lands on the stack of
  the interrupted task, overflowing the 1 KiB IDLE stack (~60% used at
  rest) and corrupting the adjacent heap.

Companion PR (nshlib improvements shown in the examples):
apache/nuttx-apps#3628

Impact

• User: the top command is now discoverable in the documentation;
  linum-stm32h753bi:nsh users get top/ps working out of the box.
• Build: none for the documentation. The defconfig adds procfs and
  CPU load measurement to one board configuration (flash: ~186 KB,
  1.7% SRAM on a 2 MB / 1 MB part).
• Hardware: linum-stm32h753bi only (defconfig change).
• Documentation: this PR is the documentation.
• Compatibility: none.

Testing

• Documentation: sphinx-build -b html completes with no warnings on
  the changed pages; new section renders at
  applications/nsh/commands.html#cmdtop.
• Defconfig: tools/configure.sh linum-stm32h753bi:nsh from distclean
  • build, then verified on hardware:

  nsh> top
  top - up 00:00:19
  Tasks: 2 total, 2 running, 0 sleeping
  %Cpu(s):  8.1 busy, 91.9 idle
  Mem :  1004876 total,     8140 used,   996736 free

    TID   PID  PPID PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK            STACK    USED FILLED    CPU COMMAND
      0     0     0   0 FIFO     Kthread   - Ready              0000000000000000 0002024 0000500  24.7%  91.8% Idle_Task
      2     2     0 100 RR       Task      - Running            0000000000000000 0004048 0001952  48.2%   8.1% nsh_main

• Without the interrupt/IDLE stack changes, enabling SCHED_CPULOAD
  on this board causes heap corruption and random asserts ("Stack
  pointer is not within the stack"), reproduced and fixed on hardware.
• tools/checkpatch.sh -g HEAD~1..HEAD: 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