📘 Document static theme development

[brianhawthorne]

Add material to the Developer Guide detailing how to build a static site against a local theme, and how to connect a step debugger to the node process.

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[brianhawthorne]

@stefanv ping

Not sure what I did to get all the --config commits appearing in this PR... but it only shows the single developer.md file as changed (which is correct). Any suggestion about how to avoid this in the future will be appreciated!

[stefanv]

Suggested change

	A theme may be deployed locally for development as a dynamic theme server, _or_ a site may be statically built against a local theme. Development procedures will be different depending on which deployment scenario you are targeting.
	For local development, there are two ways to deploy the theme (AKA the website):
	as a server, or as a static build. Development procedures are slightly different, depending on which approach you take.

I hope my rephrasing is accurate 🤔

Here, we could also add a short sentence as to why you might use which approach. E.g., something like: "The static build approach is typically used when..." where it might be "you need a React debug build" or "you want to emulate a GitHub Pages deploy" or ... ?

I just re-read the whole thing, and realized that matters are further complicated, because additionally you can build the theme into a JS package, or access the theme locally. It feels like, in this PR, we may be conflating the two axes: static/non-static build, pre-built or non-pre-built theme server.