Port all documentation to JSDoc

[abought]

Refs #186

The current documentation is a wiki, which is not tied to a specific version, and is updated by hand. It omits many layout options and is cumbersome to keep accurate/up to date. Some of the wiki info reflects features that were never fully implemented or removed, confusing users mightily.

We have been building JSDoc comments into the codebase to prepare for a transition to machine-generated API docs. Build, test, and expose those docs to replace the wiki, with improved information about layout options.

New things to document:

• Event hooks (and signatures)
• Reorganize guides and docs into a consistent structure
• Check public and private api specs
• Capture missing layout directive options (esp adapters, widgets, and layers)
  • Can we add markdown to the adapters section to explain the data chain to developers?
• Template adjustments (eg remove build date from footer)
• Provide header info and markdown sections in module docs explaining how to load an extension.
• Cross link with tutorials, to show idiomatic usage and encourage users not to interact with classes directly.
• Improve layout guide with more details for using scalable layout directives

[abought]

Core updates complete. Will merge this in order to combine with other guides in progress- it won't go live until the "working with data", "interactivity", and "rendering with layouts" tutorials are completed. (a trio of key concepts in a user-facing style, so they can get help without having to know about LZ internals first)