Improve MongoDB comparison page

[segphault]

We're going to start overhauling the content on the RethinkDB compared to MongoDB page in the documentation. The current version was written some time ago and needs an update to ensure that it properly reflects the current status of both databases. It's important that this comparison remains honest and fair, while emphasizing some of our differentiating qualities.

I'd like to collect some suggestions from the team about specific points that we might want to address in an updated comparison.

• What topics are worth adding?
• How do we view their acquisition of WiredTiger and the related storage backend improvements?
• Are we prepared to provide real-world performance comparisons and benchmarking?

The goal is to have these bullet points figured out by this Friday 8/21 and we'll update the page and go live by 8/28. Thanks.

@segphault I have a contact who used to work for Mongo who is available to help with this once we're ready to get the ball rolling on this again. He's in our Slack group so you should be able to quickly chat with him.