Documentation: document hex2bin command.

[Zepp-Hanzj]

Summary

Fill the blank hex2bin documentation page with an overview, configuration options, usage synopsis, option descriptions, examples, and notes.

All claims are sourced from the hex2bin_main.c implementation and the Kconfig in apps/system/hex2bin/.

Refs #11081

Impact

• Impact on user: YES — adds missing command documentation for the hex2bin NSH command.
• Impact on build: NO
• Impact on hardware: NO
• Impact on documentation: NO (this IS the documentation change)
• Impact on security: NO
• Impact on compatibility: NO

Testing

Built the NuttX documentation locally with Sphinx (dummy builder) — no errors or warnings for the new page:

cd Documentation && PYTHONPATH=_extensions python3 -m sphinx -b dummy . /tmp/out
# build succeeded, no hex2bin warnings

RST parsed cleanly with docutils as a fast pre-check.

PR Verification Self-Check

• I verified the hex2bin source in apps/system/hex2bin/hex2bin_main.c and its Kconfig.
• I used plain inline-literal backticks for Kconfig symbols (not :kconfig:option:).
• The PR changes exactly one .rst file.
• The Sphinx docs build passes with no new warnings.