Add confval directive for sphinx and update YCSB benches

doc/sphinx/conf.py

doc/sphinx/ext/custom.py

doc/www/content/js/bench_tabs.js

doc/www/content/ycsb/1.py

doc/www/content/ycsb/A_READ_latency.json

doc/www/content/ycsb/A_UPDATE_latency.json

doc/www/content/ycsb/A_throughput.json

doc/www/content/ycsb/B_READ_latency.json

doc/www/content/ycsb/B_UPDATE_latency.json

doc/www/content/ycsb/B_throughput.json

doc/www/content/ycsb/C_READ_latency.json

doc/www/content/ycsb/C_throughput.json

doc/www/content/ycsb/D_INSERT_latency.json

doc/www/content/ycsb/D_READ_latency.json

doc/www/content/ycsb/D_throughput.json

doc/www/content/ycsb/E_INSERT_latency.json

doc/www/content/ycsb/E_SCAN_latency.json

doc/www/content/ycsb/E_throughput.json

doc/www/content/ycsb/F_READ-MODIFY-WRITE_latency.json

doc/www/content/ycsb/F_READ_latency.json