Docusaurus migration: custom-styled blocks

[pdaoust]

On the 11ty-based site, we have some custom code blocks:

• admonitions (only one type, info -- it'd be nicer if we had the usual full palette of admonitions such as warn and tip)
• intro
• orientation
• storystep
• h-author (not sure what this is for; check to see whether it's still needed)
• topic-list
• output-block (made to suppress the copy button on code blocks)

Replicate their functionality and, if necessary, change Markdown documents to match whatever syntax they now use.