Simplify rate limits documentation in relation to pricing page

[rmglennon]

See what we can do to simplify this page and make it so we don't need to maintain this page and the pricing page

https://mapzen.com/documentation/overview/rate-limits/

• link to anchors of pricing page for each section
• (maybe) see if we can remove the free limits here for Flex services
• make free limits more obvious

(via some feedback from @souperneon)

Over time, maybe this page could be deprecated in favor of the pricing page. We'd need to find a home for the service limits (such as distance and number of points) and all services would need to be available with Flex.