Documentation: nsh: document the top command

The top command was added to nshlib in 2024 but was never documented.
Add a section to the NSH commands page covering syntax, options,
example output and configuration dependencies, including the
Linux-like summary header.

Also enable the command in the linum-stm32h753bi:nsh config: procfs,
CPU load measurement, stack coloration and task names, so that top
and ps are fully functional out of the box. This requires a dedicated
interrupt stack and larger 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 and corrupting the adjacent heap.

Also add the VT100 escape sequences used by the top command screen
refresh as string literals in include/nuttx/vt100.h (VT100_STR_*),
next to the existing VT100_FMT_* definitions.

Tested on hardware.

Signed-off-by: Jorge Guzman <jorge.gzm@gmail.com>

include/nuttx/vt100.h

@@ -225,6 +225,15 @@
 #define VT100_FMT_HVPOS      "\033[%d;%df"                    /* Move cursor to screen location v,h */
 #define VT52_FMT_CURSORPOS   "\033%d%d"                       /* Move cursor to v,h location */
 
+/* VT100 escape sequences as strings, suitable for printf-style output */
+
+#define VT100_STR_CLEARSCREEN "\033[2J"                       /* Clear entire screen */
+#define VT100_STR_CURSORHOME  "\033[1;1H"                     /* Move cursor to upper left corner */
+#define VT100_STR_CLEAREOL    "\033[K"                        /* Erase to end of current line */
+#define VT100_STR_CLEAREOS    "\033[J"                        /* Erase to end of screen */
+#define VT100_STR_BOLD        "\033[1m"                       /* Turn bold mode on */
+#define VT100_STR_MODESOFF    "\033[0m"                       /* Turn off character attributes */
+
 /* VT100 Special Key Codes
  *
  * These are sent from the terminal back to the computer when the particular