Split docs-snippet and docs-code-block?

Need to be able to render code blocks with Copy button & dark background, separate from the _content_ of those blocks. (Sometimes the content comes from a static snippet, but sometimes it comes from dynamic runtime code (e.g. import paths in autogenerated API docs)).

Start by locating the existing docs-snippet and docs-code-block entry points and tracing how their rendered content is supplied. The work is done when code-block presentation, including the Copy button and dark background, is separate from whether its content comes from a static snippet or dynamic runtime code.

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