Developers guide: A guided tour of sbt code base

[eed3si9n]

No description provided.

[dwijnand]

👍 Would be great.

[ches]

Similarly I think it might be nice to extract the plugin authoring information from the main reference to a section of the dev guide, with a good running example plugin as a walkthrough ideally. Currently the plugins section of the main reference is pretty mixed up: it contains information for plugin users as well as plugin authors, jumping around at times for both, with the distinctions between "General Best Practices" and "Plugin Best Practices" being kind of blurry.