Compare Plans!

Yesterday, during the :rooster: launch, the question I heard again and again was "how do I compare my plans?"

This morning, our qualaroo feedback continued that trend. Here are a few excerpts:

* "I want to compare my existing $25/month plan with the new Small Business $25/month plan."
* "The difference between my current plan and the new one"
* "I've an App sumo Free plan. it has less videos mat mor features, am I right?"
* "Differences between my plan and the new ones (my current plan is not listed so I had to work it out...)"

It can be pretty tough to find that in the documentation - and it shouldn't be!

This PR contains:

* remove `noindex, follow` from the plans comparison page, so it's findable on Google (we added that during :snail: launch so it wasn't weird for folks to find that page)
* Add a category and description to that page, so when it shows up in Doc search results it makes sense.
* Add a prominent link to it in our doc "Greatest Hits" and "Getting Set Up" section

There are obvi room for additions, but I think this alone will be helpful!

:bomb: