boards/linum-stm32h753bi: add curl config and document the curl command

[JorgeGzm]

Summary

Add a curl configuration for the linum-stm32h753bi board, based on the
netnsh configuration (Ethernet + DHCP). It enables the system/curl HTTP
client command, SD card support (mounted manually, as in the sdcard
configuration) and a larger console line buffer (CONFIG_LINE_MAX=256) so long
URLs and JSON bodies are not truncated at the NSH prompt.

Also document the curl command:

• a man page under Documentation/applications/system/curl/
• a usage example (download, POST JSON, multipart upload) in the board docs

Companion PR (the command itself): apache/nuttx-apps#3651

Impact

• New board configuration linum-stm32h753bi:curl; no existing configuration is
  changed.
• Otherwise documentation only (one new man page + a board-docs example).

Testing

• Build Host: Linux, arm-none-eabi-gcc
• Target: linum-stm32h753bi:curl, flashed on hardware (STM32H753BI, Ethernet + microSD)

nsh> mount -t vfat /dev/mmcsd0 /mnt
nsh> ifconfig
eth0    Link encap:Ethernet HWaddr 00:e0:de:ad:be:ef at RUNNING mtu 1486
        inet addr:192.168.15.3 DRaddr:192.168.15.1 Mask:255.255.255.0

Download to SD (-o) — text, binary and image, all HTTP 200:
nsh> curl -o /mnt/dl.json http://httpbin.org/json
curl: HTTP 200
nsh> curl -o /mnt/1kb.bin http://httpbin.org/bytes/1024
curl: HTTP 200
nsh> curl -o /mnt/img.png http://httpbin.org/image/png
curl: HTTP 200
nsh> ls -l /mnt/1kb.bin /mnt/img.png
 -rw-rw-rw-        1024 /mnt/1kb.bin
 -rw-rw-rw-        8090 /mnt/img.png

POST JSON echoed back by httpbin:
nsh> curl -H Content-Type:application/json -d '{"temp":25.3,"hum":60}' http://httpbin.org/post
{
  "json": { "hum": 60, "temp": 25.3 },
  "url": "http://httpbin.org/post"
}
curl: HTTP 200

Multipart image upload + download back (byte-identical round-trip):
nsh> curl -F file=@/mnt/nuttx_logo.png "http://192.168.15.12:8000/api/.../ota/upload?version=logo1"
{"filename":"nuttx_logo.png","sha256":"ca278dcb...","size_bytes":40343}curl: HTTP 201
nsh> curl -o /mnt/logo_back.png http://192.168.15.12:8000/api/.../ota/default/logo1/nuttx_logo.png
curl: HTTP 200
nsh> ls -l /mnt
 -rw-rw-rw-       40343 nuttx_logo.png
 -rw-rw-rw-       40343 logo_back.png

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

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• esp32-devkitc
• hifive1-revb
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• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
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