docs(geekbench): document usage, forwarding rules, and output artifacts

- Explain wrapper options vs forwarded Geekbench args
- Add examples for single/multi-core, runs, and taskset pinning
- Document generated logs/CSVs and .res semantics for LAVA

Signed-off-by: Srikanth Muppandam <smuppand@qti.qualcomm.com>

Author: smuppand