docs: Finish cleaning up and promote the API docs

[jaredcwhite]

NOTE: I now consider it absolutely imperative that v1.0 ship with 100% YARD comments…I'm not saying codebase coverage will be 100%, but rather that of the comments in the codebase, they will all be in the YARD format.

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We've been running https://api.bridgetownrb.com (now on Render instead of Netlify) for a while not, but it hasn't been publicized anywhere. That's OK for most people using Bridgetown in a basic way just to build a website, but for anyone who starts wanting to get into advanced plugin development or even really in-depth Ruby templates, they're going to need to know more about the internals of Bridgetown.

So let's get #47 a bit farther along and the officially "launch" the API docs. I think after the release of 0.21 will be a good opportunity to do that.