Documentation/nxinit: document boot reason property and compound cmds

[JianyuWang0623]

Summary

Reviewers on apps#3751 (support compound command and resetcause-based triggers) asked for documentation of the new features. This adds two sections to Documentation/applications/system/nxinit/index.rst:

• Built-in Properties: describes the sys.boot.reason property set by NXInit at startup from BOARDIOC_RESET_CAUSE, its two value forms (hardware cause with numeric subreason, or software reset reason string), and the behavior when CONFIG_BOARDCTL_RESET_CAUSE is disabled or the boardctl() call fails.
• Compound Commands: describes the && / || short-circuit semantics for chaining commands on a single action line, including quoting behavior.

Impact

Documentation only. No code, config, or behavior changes.

Testing

Build Host: Linux x86_64, Sphinx 8.1.3, Python 3

Built the applications/system/nxinit doc page standalone with the repo's Sphinx config:

$ python3 -m sphinx -b html -c . applications/system/nxinit /tmp/nxinit_docbuild
Running Sphinx v8.1.3
...
build succeeded.
The HTML pages are in .../tmp/nxinit_docbuild.

No RST syntax/build errors. Also ran tools/checkpatch.sh -f on the changed file:

$ ./tools/checkpatch.sh -f Documentation/applications/system/nxinit/index.rst
✔️ All checks pass.

Marked as draft pending review from @linguini1 and @cederom on the original request in apps#3751.