Content: Pricing page intro to our plans and platforms

[nienn]

At the moment the pricing page feels a bit like a collection of links to somewhere else and it's not very informative about the different options the user can choose and the main benefits, focus or intended target of each option.

The message in this page could be re-worked to be more informative, so that the user can reach the end with a clearer grasp of the different options provided by Read the Docs.

Related discussion: #34 (comment)

[agjohnson]

See also #65 for some of the FAQ content that might be good to add/move to our docs.