Documentation: document lzf command.

[Zepp-Hanzj]

Summary

• Add documentation for the lzf compression/decompression command, which currently has a blank documentation page.
• The lzf tool uses the liblzf library to compress and decompress files using the lightweight LZF algorithm.
• The documentation covers: overview, file format, configuration (CONFIG_SYSTEM_LZF and related symbols), usage, all six options (-c, -d, -f, -h, -v, -b), environment variable support (LZF_BLOCKSIZE), examples for file and stdin/stdout modes, and implementation notes.
• Refs Documentation: blank pages in documentation #11081

Impact

• Is new feature added? Is existing feature changed? NO
• Impact on user (will user need to adapt to change)? NO
• Impact on build (will build process change)? NO
• Impact on hardware (will arch(s) / board(s) / driver(s) change)? NO
• Impact on documentation (is update required / provided)? YES — this PR is the documentation update itself.
• Impact on security (any sort of implications)? NO
• Impact on compatibility (backward/forward/interoperability)? NO

Testing

• Build Host(s): Linux (WSL Ubuntu), x86_64
• Verification: python3 -m docutils RST syntax check — no warnings. make html in Documentation/ — build succeeded with no errors.
• ./tools/checkpatch.sh -g HEAD — all checks pass.
• Source evidence: documentation content derived from apps/system/lzf/lzf_main.c (578 lines) and apps/system/lzf/Kconfig, cross-referenced with include/lzf.h for file format constants.