Summary:
Changed leveldb documentation with rocksdb in doc/index.html. Added some of the important options from options.h to doc.

Also removed benchmark files and impl.h, since this is all replaced by RocksDB wikis.

Test Plan: -

Reviewers: dhruba, haobo, kailiu, emayanke, sdong

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13977