Documentation/applications/system/nxinit: document the "console" option

[JianyuWang0623]

Summary

Documents the per-service console [<device>] option added to nxinit
by the companion apache/nuttx-apps#3776.

Documentation/applications/system/nxinit/index.rst is nxinit's only
existing doc page. This adds console to the option list in "Services
and Options" and a dedicated subsection explaining:

• what the option does (open the given device, or
  CONFIG_SYSTEM_NXINIT_CONSOLE_DEV if omitted, and dup it onto the
  service's stdin/stdout/stderr before spawning);
• why a plain shell service needs it while nsh does not (nsh
  already does the equivalent internally via nsh_consolemain());
• why a USB gadget console (CDC-ACM/PL2303) additionally needs the
  gadget itself brought up first (e.g. via exec -- sercon), since
  the device does not exist until then, unlike a plain UART.

Impact

Documentation only, no code change.

Testing

$ tools/checkpatch.sh -g HEAD
✔️ All checks pass.

$ cd Documentation && make html
...
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded.

The HTML pages are in _build/html.

Confirmed the new section renders correctly in the generated output
(_build/html/applications/system/nxinit/index.html):

$ grep -o "console.\{0,80\}" _build/html/applications/system/nxinit/index.html
console-option">The <code class="docutils literal notranslate"><span class="pre">consol
console (redirect the service's stdio to a console device; see below),
console-option">
console</span></code> Option<a class="headerlink" href="#the-console-option" title="Lin
console [ &lt;device&gt; ]