Gear Ratios page receives major overhaul

Many gearing options have become available since the Gear Ratios page was written in 2016 that make it easier to get lower gearing on road bikes. This includes super-compact gravel cranksets, 12-speed systems, and single-chainring setups. I therefore re-wrote the page to be clearer and include all of the latest options including extensive tables of all useful cassettes.