Documentation: Document dd command

[Zepp-Hanzj]

Summary

• Add documentation for the dd command.
• The command copies data from an input file or device to an output file or device.
• Covers overview, configuration, usage, options, examples, and notes.
• Related NuttX Issue: Documentation: blank pages in documentation #11081

Impact

• Is new feature added? NO.
• Is existing feature changed? NO.
• Impact on user? NO.
• Impact on build? NO.
• Impact on hardware? NO.
• Impact on documentation? YES: adds documentation for the dd command.
• Impact on security? NO.
• Impact on compatibility? NO.
• Dependencies: none.

Testing

I confirm that changes are verified on local setup and work as intended:

• Build Host(s): Linux WSL2
• Target(s): Documentation build only
• Static/style check: RST syntax validation
• Build test: cd Documentation && make html

Testing logs after change:

$ cd Documentation && DISPLAY= JAVA_TOOL_OPTIONS='-Djava.awt.headless=true' make html
build succeeded.
The HTML pages are in _build/html.

PR verification Self-Check

• This PR introduces only one functional change.
• I have updated all required description fields above.
• My PR adheres to Contributing Guidelines and Documentation (git commit title and message, coding standard, etc).
• My PR is still work in progress (not ready for review).
• My PR is ready for review and can be safely merged into a codebase.